Simplified HPLC method for urinary and circulating creatinine

Mass Spectrometry-Based Evaluation of the Bland-Altman Approach: Review, Discussion, and Proposal

Reliable quantification in biological systems of endogenous low- and high-molecular substances, drugs and their metabolites, is of particular importance in diagnosis and therapy, and in basic and clinical research. The analytical characteristics of analytical approaches have many differences, including in core features such as accuracy, precision, specificity, and limits of detection (LOD) and quantitation (LOQ). Several different mathematic approaches were developed and used for the comparison of two analytical methods applied to the same chemical compound in the same biological sample. Generally, comparisons of results obtained by two analytical methods yields different quantitative results. Yet, which mathematical approach gives the most reliable results? Which mathematical approach is best suited to demonstrate agreement between the methods, or the superiority of an analytical method A over analytical method B? The simplest and most frequently used method of comparison is the linear regression analysis of data observed by method A (y) and the data observed by method B (x): y = α + βx. In 1986, Bland and Altman indicated that linear regression analysis, notably the use of the correlation coefficient, is inappropriate for method-comparison. Instead, Bland and Altman have suggested an alternative approach, which is generally known as the Bland-Altman approach. Originally, this method of comparison was applied in medicine, for instance, to measure blood pressure by two devices. The Bland-Altman approach was rapidly adapted in analytical chemistry and in clinical chemistry. To date, the approach suggested by Bland-Altman approach is one of the most widely used mathematical approaches for method-comparison. With about 37,000 citations, the original paper published in the journal The Lancet in 1986 is among the most frequently cited scientific papers in this area to date. Nevertheless, the Bland-Altman approach has not been really set on a quantitative basis. No criteria have been proposed thus far, in which the Bland-Altman approach can form the basis on which analytical agreement or the better analytical method can be demonstrated. In this article, the Bland-Altman approach is re-valuated from a quantitative bioanalytical perspective, and an attempt is made to propose acceptance criteria. For this purpose, different analytical methods were compared with Gold Standard analytical methods based on mass spectrometry (MS) and tandem mass spectrometry (MS/MS), i.e., GC-MS, GC-MS/MS, LC-MS and LC-MS/MS. Other chromatographic and non-chromatographic methods were also considered. The results for several different endogenous substances, including nitrate, anandamide, homoarginine, creatinine and malondialdehyde in human plasma, serum and urine are discussed. In addition to the Bland-Altman approach, linear regression analysis and the Oldham-Eksborg method-comparison approaches were used and compared. Special emphasis was given to the relation of difference and mean in the Bland-Altman approach. Currently available guidelines for method validation were also considered. Acceptance criteria for method agreement were proposed, including the slope and correlation coefficient in linear regression, and the coefficient of variation for the percentage difference in the Bland-Altman and Oldham-Eksborg approaches.

Effects of Low Doses of L-Carnitine Tartrate and Lipid Multi-Particulate Formulated Creatine Monohydrate on Muscle Protein Synthesis in Myoblasts and Bioavailability in Humans and Rodents

The primary objective of this study was to investigate the potential synergy between low doses of L-carnitine tartrate and creatine monohydrate to induce muscle protein synthesis and anabolic pathway activation in primary human myoblasts. In addition, the effects of Lipid multi-particulates (LMP) formulation on creatine stability and bioavailability were assessed in rodents and healthy human subjects. When used individually, L-carnitine tartrate at 50 µM and creatine monohydrate at 0.5 µM did not affect myoblast protein synthesis and signaling. However, when combined, they led to a significant increase in protein synthesis. Increased AKT and RPS6 phosphorylation were observed with 50 µM L-carnitine tartrate 5 µM creatine in combination in primary human myoblasts. When Wistar rats were administered creatine with LMP formulation at either 21 or 51 mg/kg, bioavailability was increased by 27% based on the increase in the area under the curve (AUC) at a 51 mg/kg dose compared to without LMP formulation. Tmax and Cmax were unchanged. Finally, in human subjects, a combination of LMP formulated L-carnitine at 500 mg (from L-carnitine tartrate) with LMP formulated creatine at 100, 200, or 500 mg revealed a significant and dose-dependent increase in plasma creatine concentrations. Serum total L-carnitine levels rose in a similar manner in the three combinations. These results suggest that a combination of low doses of L-carnitine tartrate and creatine monohydrate may lead to a significant and synergistic enhancement of muscle protein synthesis and activation of anabolic signaling. In addition, the LMP formulation of creatine improved its bioavailability. L-carnitine at 500 mg and LMP-formulated creatine at 200 or 500 mg may be useful for future clinical trials to evaluate the effects on muscle protein synthesis.

GC-MS Studies on Derivatization of Creatinine and Creatine by BSTFA and Their Measurement in Human Urine

In consideration of its relatively constant urinary excretion rate, creatinine (2-amino-1-methyl-5H-imidazol-4-one, MW 113.1) in urine is a useful endogenous biochemical parameter to correct the urinary excretion rate of numerous endogenous and exogenous substances. Reliable measurement of creatinine by gas chromatography (GC)-based methods requires derivatization of its amine and keto groups. Creatinine exists in equilibrium with its open form creatine (methylguanidoacetic acid, MW 131.1), which has a guanidine and a carboxylic group. Trimethylsilylation and trifluoroacetylation of creatinine and creatine are the oldest reported derivatization methods for their GC-mass spectrometry (MS) analysis in human serum using flame- or electron-ionization. We performed GC-MS studies on the derivatization of creatinine (d₀-creatinine), [methylo-²H₃]creatinine (d₃-creatinine, internal standard) and creatine (d₀-creatine) with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) using standard derivatization conditions (60 min, 60 °C), yet in the absence of any base. Reaction products were characterized both in the negative-ion chemical ionization (NICI) and in the positive-ion chemical ionization (PICI) mode. Creatinine and creatine reacted with BSTFA to form several derivatives. Their early eluting N,N,O-tris(trimethylsilyl) derivatives (8.9 min) were found to be useful for the precise and accurate measurement of the sum of creatinine and creatine in human urine (10 µL, up to 20 mM) by selected-ion monitoring (SIM) of m/z 271 (d₀-creatinine/d₀-creatine) and m/z 274 (d₃-creatinine) in the NICI mode. In the PICI mode, SIM of m/z 256, m/z 259, m/z 272 and m/z 275 was performed. BSTFA derivatization of d₀-creatine from a freshly prepared solution in distilled water resulted in formation of two lMate-eluting derivatives (14.08 min, 14.72 min), presumably creatinyl-creatinine, with the creatininyl residue existing in its enol form (14.08 min) and keto form (14.72 min). Our results suggest that BSTFA derivatization does not allow specific analysis of creatine and creatinine by GC-MS. Preliminary analyses suggest that pentafluoropropionic anhydride (PFPA) is also not useful for the measurement of creatinine in the presence of creatine. Both BSTFA and PFPA facilitate the conversion of creatine to creatinine. Specific measurement of creatinine in urine is possible by using pentafluorobenzyl bromide in aqueous acetone.

Local and Systemic Alterations of the L-Arginine/Nitric Oxide Pathway in Sputum, Blood, and Urine of Pediatric Cystic Fibrosis Patients and Effects of Antibiotic Treatment

Alterations in the L-arginine (Arg)/nitric oxide (NO) pathway have been reported in cystic fibrosis (CF; OMIM 219700) as the result of various factors including systemic and local inflammatory activity in the airways. The aim of the present study was to evaluate the Arg/NO metabolism in pediatric CF patients with special emphasis on lung impairment and antibiotic treatment. Seventy CF patients and 78 healthy controls were included in the study. CF patients (43% male, median age 11.8 years) showed moderately impaired lung functions (FEV1 90.5 ± 19.1% (mean ± SD); 21 (30%) had a chronic Pseudomonas aeruginosa (PSA) infection, and 24 (33%) had an acute exacerbation). Plasma, urinary, and sputum concentrations of the main Arg/NO metabolites, nitrate, nitrite, Arg, homoarginine (hArg), and asymmetric dimethylarginine (ADMA) were determined in pediatric CF patients and in healthy age-matched controls. Clinical parameters in CF patients included lung function and infection with PSA. Additionally, the Arg/NO pathway in sputum samples of five CF patients was analyzed before and after routine antibiotic therapy. CF patients with low fractionally exhaled NO (FENO) showed lower plasma Arg and nitrate concentrations. During acute exacerbation, sputum Arg and hArg levels were high and dropped after antibiotic treatment: Arg: pre-antibiotics: 4.14 nmol/25 mg sputum vs. post-antibiotics: 2.33 nmol/25 mg sputum, p = 0.008; hArg: pre-antibiotics: 0.042 nmol/25 mg sputum vs. post-antibiotics: 0.029 nmol/25 mg sputum, p = 0.035. The activated Arg/NO metabolism in stable CF patients may be a result of chronic inflammation. PSA infection did not play a major role regarding these differences. Exacerbation increased and antibiotic therapy decreased sputum Arg concentrations.

Measurement of serum creatinine levels with liquid chromatography-tandem mass spectrometry: comparison with Jaffe and enzymatic methods

Objectives

Our aim was to validate a mass spectrometric creatinine method and compare this method with Jaffe and enzymatic serum creatinine methods.

Methods

90 samples were included. The levels were classified into three groups according to serum creatinine results as Group 1: Lower (n=30) (0.16–0.59 mg/dL), Group 2: Normal (n=30) (0.62–1.18 mg/dL) and Group 3: Higher (n=30) (1.33–3.88 mg/dL). Jaffe and enzymatic creatinine measurements were performed on the Beckman Coulter AU5800 autoanalyzer.

Results

Serum creatinine was linear from 0.039 up to 10 mg/dL, CV and bias values were ranged between 1.9–3.8% and 2–15%. Correlation coefficients were 0.990 (95% confidence interval 0.984–0.993), 0.992 (95% confidence interval 0.988–0.995) and 0.994 (95% confidence interval 0.991–0.996) for LC-MS/MS-Enzymatic, LC-MS/MS-Jaffe and Enzymatic-Jaffe, respectively.

Conclusions

Although, Jaffe method for serum creatinine measurement is still much more practical and cheap, so in use for routine practice, tandem mass spectrometric detection of serum creatinine can be used as an accurate and specific method for verification of discordant clinical results, existence of possible interferences and serum levels under 0.5 mg/dL creatinine results such as pediatric or pregnant populations.

Effects of Cognitive Behavioral Therapy for Insomnia on Sleep, Symptoms, Stress, and Autonomic Function Among Patients With Heart Failure

Background: Insomnia is common among patients with stable heart failure (HF) and associated with inflammation and altered autonomic function. Purpose: The purposes of this study were to examine the effects of cognitive behavioral therapy for insomnia (CBT-I) on the Hypothalamic Pituitary (HPA) Axis, autonomic function, inflammation, and circadian rhythmicity and the associations between these biomarkers and insomnia, sleep characteristics, symptoms, functional performance, and sleep-related cognitions. Methods: We conducted a subanalysis of a pilot randomized controlled trial (RCT, NCT02827799) whose primary aim was to test the effects of CBT-I on insomnia. We randomized 51 patients with stable Class II-IV HF to CBT-I (n = 30) or attention control (n = 21). Participants completed wrist actigraphy and self-reported insomnia severity, sleep characteristics, sleep-related cognitions, daytime symptoms, and functional performance. We measured day and nighttime urinary free cortisol, melatonin sulfate, epinephrine, and norepinephrine at baseline, and two weeks after CBT-I and computed general linear models and partial correlations. Results: CBT-I had no effects on the biomarkers, but there were statistically significant negative cross-sectional correlations between the ratio of day and night urinary free cortisol and sleep disturbance, anxiety, fatigue, depression, and negative sleep cognitions. Increases in the ratio between day and night cortisol were associated with statistically significant improvements in fatigue, depression, sleep duration, and sleep-related cognitions. Conclusions: Biomarkers of stress and autonomic function are associated with sleep, sleep-related symptoms, and cognitions among people with chronic HF. Future studies are needed to identify potential causal relationships and the impact of sleep interventions.

Impaired mitochondrial maturation of sulfite oxidase in a patient with severe sulfite oxidase deficiency

Sulfite oxidase (SO) is encoded by the nuclear SUOX gene and catalyzes the final step in cysteine catabolism thereby oxidizing sulfite to sulfate. Oxidation of sulfite is dependent on two cofactors within SO, a heme and the molybdenum cofactor (Moco), the latter forming the catalytic site of sulfite oxidation. SO localizes to the intermembrane space of mitochondria where both—pre-SO processing and cofactor insertion—are essential steps during SO maturation. Isolated SO deficiency (iSOD) is a rare inborn error of metabolism caused by mutations in the SUOX gene that lead to non-functional SO. ISOD is characterized by rapidly progressive neurodegeneration and death in early infancy. We diagnosed an iSOD patient with homozygous mutation of SUOX at c.1084G>A replacing Gly362 to serine. To understand the mechanism of disease, we expressed patient-derived G362S SO in Escherichia coli and surprisingly found full catalytic activity, while in patient fibroblasts no SO activity was detected, suggesting differences between bacterial and human expression. Moco reconstitution of apo-G362S SO was found to be approximately 90-fold reduced in comparison to apo-WT SO in vitro. In line, levels of SO-bound Moco in cells overexpressing G362S SO were significantly reduced compared to cells expressing WT SO providing evidence for compromised maturation of G362S SO in cellulo. Addition of molybdate to culture medium partially rescued impaired Moco binding of G362S SO and restored SO activity in patient fibroblasts. Thus, this study demonstrates the importance of the orchestrated maturation of SO and provides a first case of Moco-responsive iSOD.

Development and validation of an ultra-high performance liquid chromatography-tandem mass spectrometry method for the simultaneous determination of iohexol, p-aminohippuric acid and creatinine in porcine and broiler chicken plasma

In order to study the renal function, in terms of glomerular filtration and effective renal plasma flow, in broiler chickens and pigs, an ultra-high performance liquid chromatography-tandem mass spectrometry method for the simultaneous determination of iohexol, p-aminohippuric acid (PAH) and exogenously administered creatinine in plasma was developed and validated. Sample preparation consisted of a deproteinization step using methanol for porcine plasma and an Ostro™ Protein Precipitation & Phospholipid Removal Plate was used for broiler chicken plasma. Chromatographic separation was achieved on a Hypersil Gold aQ column using 0.1% formic acid in water and 0.1% formic acid in methanol as mobile phases. The total run time was limited to 10 min. Matrix-matched calibration curves for iohexol and PAH were prepared and good linearity (r ≥ 0.9973; gof ≤ 6.17%) was achieved over the concentration range tested (0.25-90 μg/mL). Limits of quantification were 0.25 μg/mL for iohexol and PAH. Water was used as surrogate matrix for analysis of creatinine in plasma. This surrogate calibration curve showed good linearity over the concentration range tested (0.25-90 μg/mL) (r ≥ 0.9979; gof ≤ 5.66%). For creatinine, the relative lower limit of quantification was 201.03 ± 49.20% and 60.14 ± 7.64% for chicken and porcine plasma, respectively. The results for within-day and between-day precision and accuracy fell within the specified ranges. This straightforward, cost-effective and rapid method, determining iohexol, PAH and creatinine within one single chromatographic run, has been successfully used for the analysis in porcine and broiler chicken plasma samples in order to determine the renal function of these species.

Realising the Potential of Urine and Saliva as Diagnostic Tools in Sport and Exercise Medicine

Accurate monitoring of homeostatic perturbations following various psychophysiological stressors is essential in sports and exercise medicine. Various biomarkers are routinely used as monitoring tools in both clinical and elite sport settings. Blood collection and muscle biopsies, both invasive in nature, are considered the gold standard for the analysis of these biomarkers in exercise science. Exploring non-invasive methods of collecting and analysing biomarkers that are capable of providing accurate information regarding exercise-induced physiological and psychological stress is of obvious practical importance. This review describes the potential benefits, and the limitations, of using saliva and urine to ascertain biomarkers capable of identifying important stressors that are routinely encountered before, during, or after intense or unaccustomed exercise, competition, over-training, and inappropriate recovery. In particular, we focus on urinary and saliva biomarkers that have previously been used to monitor muscle damage, inflammation, cardiovascular stress, oxidative stress, hydration status, and brain distress. Evidence is provided from a range of empirical studies suggesting that urine and saliva are both capable of identifying various stressors. Although additional research regarding the efficacy of using urine and/or saliva to indicate the severity of exercise-induced psychophysiological stress is required, it is likely that these non-invasive biomarkers will represent "the future" in sports and exercise medicine.

Methods for Quantitative Creatinine Determination

Reliable measurement of creatinine is necessary to assess kidney function, and also to quantitate drug levels and diagnostic compounds in urine samples. The most commonly used methods are based on the Jaffe principal of alkaline creatinine-picric acid complex color formation. However, other compounds commonly found in serum and urine may interfere with Jaffe creatinine measurements. Therefore, many laboratories have made modifications to the basic method to remove or account for these interfering substances. This appendix will summarize the basic Jaffe method, as well as a modified, automated version. Also described is a high performance liquid chromatography (HPLC) method that separates creatinine from contaminants prior to direct quantification by UV absorption. Lastly, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method is described that uses stable isotope dilution to reliably quantify creatinine in any sample. This last approach has been recommended by experts in the field as a means to standardize all quantitative creatinine methods against an accepted reference. © 2017 by John Wiley & Sons, Inc.

Nanocoating cellulose paper based microextraction combined with nanospray mass spectrometry for rapid and facile quantitation of ribonucleosides in human urine

A rapid and facile analytical method for quantification of ribonucleosides in human urine was developed by the combination of nanocoating cellulose paper based microextraction and nanoelectrospray ionization-tandem mass spectrometry (nESI-MS/MS). Cellulose paper used for microextraction was modified by nano-precision deposition of uniform ultrathin zirconia gel film using a sol-gel process. Due to the large surface area of the cellulose paper and the strong affinity between zirconia and the cis-diol compounds, the target analytes were selectively extracted from the complex matrix. Thus, the detection sensitivity was greatly improved. Typically, the nanocoating cellulose paper was immersed into the diluted urine for selective extraction of target analytes, then the extracted analytes were subjected to nESI-MS/MS detection. The whole analytical procedure could be completed within 10min. The method was evaluated by the determination of ribonucleosides (adenosine, cytidine, uridine, guanosine) in urine sample. The signal intensities of the ribonuclesides extracted by the nanocoating cellulose paper were greatly enhanced by 136-459-folds compared with the one of the unmodified cellulose paper based microextraction. The limits of detection (LODs) and the limits of quantification (LOQs) of the four ribonucleosides were in the range of 0.0136-1.258μgL^-1 and 0.0454-4.194μgL^-1, respectively. The recoveries of the target nucleosides from spiked human urine were in the range of 75.64-103.49% with the relative standard deviations (RSDs) less than 9.36%. The results demonstrate the potential of the proposed method for rapid and facile determination of endogenous ribonucleosides in urine sample.

The Indicator Amino Acid Oxidation Method with the Use of l-[1-13C]Leucine Suggests a Higher than Currently Recommended Protein Requirement in Children with Phenylketonuria

BACKGROUND

Phenylketonuria is characterized by mutations in the Phe hydroxylase gene that leads to the accumulation of Phe in plasma and the brain. The standard of care for phenylketonuria is nutritional management with dietary restriction of Phe and the provision of sufficient protein and energy for growth and health maintenance. The protein requirement in children with phenylketonuria is empirically determined based upon phenylketonuria nutritional guidelines that are adjusted individually in response to biochemical markers and growth.

OBJECTIVE

We determined dietary protein requirements in children with phenylketonuria with the use of the indicator amino acid oxidation (IAAO) technique, with l-[1-¹³C]Leu as the indicator amino acid.

METHODS

Four children (2 males; 2 females) aged 9-18 y with phenylketonuria [mild hyperphenylalanemia (mHPA); 6-10 mg/dL (360-600 μmol/L)] were recruited to participate in ≥7 separate test protein intakes (range: 0.2-3.2 g ⋅ kg^-1 ⋅ d^-1) with the IAAO protocol with the use of l-[1-¹³C]Leu followed by the collection of breath and urine samples over 8 h. The diets were isocaloric and provided energy at 1.7 times the resting energy expenditure. Protein was provided as a crystalline amino acid mixture based on an egg protein pattern, except Phe and Leu, which were maintained at a constant across intakes. Protein requirement was determined with the use of a 2-phase linear-regression crossover analysis of the rate of l-[1-¹³C]Leu tracer oxidation.

RESULTS

The mean protein requirement was determined to be 1.85 g ⋅ kg^-1 ⋅ d^-1 (R² = 0.66; 95% CI: 1.37, 2.33). This result is substantially higher than the 2014 phenylketonuria recommendations (1.14-1.33 g ⋅ kg^-1 ⋅ d^-1; based on 120-140% above the current RDA for age).

CONCLUSIONS

To our knowledge, this is the first study to directly define a quantitative requirement for protein intake in children with mHPA and indicates that current protein recommendations in children with phenylketonuria may be insufficient. This trial was registered at clinicaltrials.gov as NCT01965691.

Dietary and inhalation exposure to polycyclic aromatic hydrocarbons and urinary excretion of monohydroxy metabolites--a controlled case study in Beijing, China

Daily dietary and inhalation exposures to 16 parent polycyclic aromatic hydrocarbons (PAHs) and urinary excretion of 13 monohydroxy metabolites (OHPAHs) were monitored for 12 non-smoking university students in Beijing, China, during a controlled feeding experiment. The relationship between the urinary excretion of OHPAHs and the uptake of PAHs was investigated. The results suggest severe exposure of the subjects to PAHs via both dietary and inhalation pathways. Large increase of most urinary OHPAHs occurred after the ingestion of lamb kabob. Higher concentrations of OHPAHs were observed for female subjects, with the intakes of parent PAHs lower than those by males, likely due to the gender differences in metabolism. It appears that besides 1-PYR, metabolites of PHE could also be used as biomarkers to indicate the short-term dietary exposure to PAHs and urinary 3-BaA may serve as the biomarker for inhalation intake of high molecular weight PAHs.

Quantification of creatinine in biological samples based on the pseudoenzyme activity of copper-creatinine complex

Glomerular filtration rate (GFR), the marker of chronic kidney disease can be analyzed by the concentration of cystatin C or creatinine and its clearance in human urine and serum samples. The determination of cystatin C alone as an indicator of GFR does not provide high accuracy, and is more expensive, thus measurement of creatinine has an important role in estimating GFR. We have made an attempt to quantify creatinine based on its pseudoenzyme activity of creatinine in the presence of copper. Creatinine in the presence of copper oxidizes paraphenylenediamine dihydrochloride (PPDD) which couples with dimethylamino benzoicacid (DMAB) giving green colored chromogenic product with maximum absorbance at 710 nm. Kinetic parameters relating this reaction were evaluated. Analytical curves of creatinine by fixed time and rate methods were linear at 8.8-530 μmol L(-1) and 0.221-2.65 mmol L(-1), respectively. Recovery of creatinine varied from 97.8 to 107.8%. Limit of detection and limit of quantification were 2.55 and 8.52 μmol L(-1) respectively whereas Sandell's sensitivity and molar absorption coefficient values were 0.0407 μg cm(-2) and 0.1427×10(4) L mol(-1) cm(-1) respectively. Precision studies showed that within day imprecision was 0.745-1.26% and day-to-day imprecision was 1.55-3.65%. The proposed method was applied to human urine and serum samples and results were validated in accordance with modified Jaffe's procedure. Wide linearity ranges with good recovery, less tolerance from excipients and application of the method to serum and urine samples are the claims which ascertain much advantage to this method.

LC-MS-MS Measurements of Urinary Creatinine and the Application of Creatinine Normalization Technique on Cotinine in Smokers' 24 Hour Urine

A simple and sensitive high performance liquid chromatography-tandem mass spectrometry (HPLC-ESI-MS-MS) method was developed and validated for the quantification of creatinine in human urine. The analysis was carried out on an Agilent Zorbax Eclipse XDB-C18 column (2.1 × 150 mm, 3.5 μm). The mobile phase was 0.1% formic acid in water and 0.1% formic acid in acetonitrile (50/50, v/v). Linear calibration curves were obtained in the concentration range of 1-2000.0 ng/mL, with a lower limit of quantification of 0.99 ng/mL. The intra- and interday precision (RSD) values were below 3%. The method was successfully applied to a bioequivalence study of creatinine in Chinese smokers and nonsmokers. The total cotinine in 24 h urine and cotinine : creatinine ratio were also positively associated (Pearson R = 0.942, P < 0.0001). However, cotinine : creatinine ratio varied significantly across smoking groups for the difference of individual. 24 h urinary cotinine was more appropriate for expressing correlation with tar than cotinine : creatinine ratio.

Determination of zoledronic acid in human urine and blood plasma using liquid chromatography/electrospray mass spectrometry

A new method for the analysis of 1-hydroxy-2-imidazol-1-yl-phosphonoethyl phosphoric acid (zoledronic acid) in urine and blood samples has been developed. It consists of a derivatisation of the bisphosphonate with trimethylsilyl diazomethane under multiple methylester formation. The formed derivative can, in contrast to the non-derivatised analyte, easily be separated by reversed phase liquid chromatography due to its reduced polarity. Detection is performed by electrospray tandem mass spectrometry. For calibration purposes, a deuterated internal standard has been synthesised in a three-step synthesis starting with d(4)-imidazole. For human urine, the limit of detection (LOD) is 1.2x10(-7) mol/L, limit of quantification (LOQ) is 3.75×10(-7) mol/L in the MRM mode. For human blood plasma, a LOD of 1×10(-7) mol/L and a LOQ of 2.5×10(-7) mol/L were determined. The linear dynamic range comprised 3.5 decades starting at the limit of quantification. The method was successfully applied for the analysis of spiked urine and blood plasma samples as well as samples from two osteoporosis patients.

Asymmetric dimethylarginine in children with homocystinuria or phenylketonuria

Plasma concentration of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis from L-arginine and a cardiovascular risk factor, was found to be elevated in plasma of homocysteinemic adults. Enhanced cardiovascular risk due to homocystinuria and impaired renal function has been found in patients with phenylketonuria (PKU) on protein-restricted diet. However, it is still unknown whether ADMA synthesis is also elevated in children with homocystinuria due to cystathionine beta-synthase deficiency (classical homocystinuria), and whether ADMA may play a role in phenylketonuria in childhood. In the present study, we investigated the status of the L-arginine/NO pathway in six young patients with homocystinuria, in 52 young phenylketonuria patients on natural protein-restricted diet, and in age- and gender-matched healthy children serving as controls. ADMA in plasma and urine was determined by GC-MS/MS. The NO metabolites nitrate and nitrite in plasma and urine, and urinary dimethylamine (DMA), the dimethylarginine dimethylaminohydrolase (DDAH) metabolite of ADMA, were measured by GC-MS. Unlike urine ADMA excretion, plasma ADMA concentration in patients with homocystinuria was significantly higher than in controls (660±158 vs. 475±77 nM, P=0.035). DMA excretion rate was considerably higher in children with homocystinuria as compared to controls (62.2±24.5 vs. 6.5±2.9 μmol/mmol creatinine, P=0.068), indicating enhanced DDAH activity in this disease. In contrast and unexpectedly, phenylketonuria patients had significantly lower ADMA plasma concentrations compared to controls (512±136 vs. 585±125 nM, P=0.009). Phenylketonuria patients and controls had similar L-arginine/ADMA molar ratios in plasma. Urinary nitrite excretion was significantly higher in phenylketonuria as compared to healthy controls (1.7±1.7 vs. 0.7±1.2 μmol/mmol creatinine, P=0.003). Our study shows that the L-arginine/NO pathway is differently altered in children with phenylketonuria and homocystinuria. Analogous to hyperhomocysteinemic adults, elevated ADMA plasma concentrations could be a cardiovascular risk factor in children with homocystinuria. In phenylketonuria, the L-arginine/NO pathway seems not be altered. Delineation of the role of ADMA in childhood phenylketonuria and homocystinuria demands further investigation.

Simultaneous measurement of [15N]nitrate and [15N]nitrite enrichment and concentration in urine by gas chromatography mass spectrometry as pentafluorobenzyl derivatives

Quantification of (15)N-labeled nitrate and determination of the (15)N-enrichment in urine upon administration of (15)N-labeled precursors such as L-[guanidino-(15)N(2)]arginine is a suitable approach to study formation and metabolism of nitric oxide (NO) and its metabolites in vivo. Previously, we have reported on the simultaneous derivatization and accurate quantification of nitrite and nitrate in various biological fluids using pentafluorobenzyl bromide and GC/MS. We report here on a modification of this method that allows for the simultaneous determination of (15)N-enrichment of [(15)N]nitrate and [(15)N]nitrite and the simultaneous quantification of [(15)N]nitrate, [(14)N]nitrate, [(15)N]nitrite, and [(14)N]nitrite in human urine. In a pilot study, using the carbonic anhydrase inhibitor drug acetazolamide at therapeutical oral doses (5.4 and 5 mg per kg bodyweight) and by oral intake of [(15)N]nitrite (0.31 and 0.5 micromol per kg bodyweight) by two healthy volunteers, we demonstrate for the first time that renal carbonic anhydrase activity is mainly responsible for the reabsorption of nitrite from the primary urine and confirm previous findings on nitrate.

Effects of angiotensin type 1 receptor blockade on arginine and ADMA synthesis and metabolic pathways in fawn-hooded hypertensive rats

BACKGROUND

The fawn-hooded hypertensive (FHH) rat develops spontaneous glomerulosclerosis that is ameliorated by inhibition of the angiotensin II type 1 receptor (AT-1). Since kidney damage is associated with nitric oxide (NO) deficiency, we investigated how AT-1 antagonism influenced nitric oxide synthase (NOS), as well as NOS substrate [L-arginine (L-Arg)] and inhibitor [asymmetric dimethylarginine (ADMA)]. L-Arg is synthesized by renal argininosuccinate synthase/argininosuccinate lyase (ASS/ASL) and then either consumed within the kidney by arginase II or NOS or released into the circulation. L-Arg is then taken up from plasma into cells where it can be utilized by NOS and other pathways. The competitive inhibitor of NOS, ADMA, is degraded by dimethylarginine dimethylaminohydrolase (DDAH).

METHODS AND RESULTS

Male FHH rats were put on a 40% casein diet for 13 weeks, and some received AT-1 antagonist which reduced blood pressure and kidney weight and prevented glomerulosclerosis and hyperfiltration. The AT-1 antagonist reduced the expression of DDAH2, increased DDAH1 and increased total DDAH activity in the kidney cortex, although there was no change in plasma or kidney cortex ADMA levels. The AT-1 antagonist caused no change in the expression of renal ASS/ASL, but reduced renal and aortic arginase expression and renal arginase activity, which could explain the increased plasma L-Arg. In separate studies, 1 week of AT-1 blockade in young FHH rats had no effect on any of these parameters.

CONCLUSION

Thus, the net result of AT-1 antagonist was an improved L-Arg to ADMA ratio due to the prevention of renal and vascular arginase activation which favours increased NO production. Since 1 week of AT-1 blockade in the absence of kidney damage was without effect on arginases, this suggests that the reduction in arginase activity is secondary to the prevention of structural damage rather than a direct immediate effect of AT-1 antagonism.

GC-MS determination of creatinine in human biological fluids as pentafluorobenzyl derivative in clinical studies and biomonitoring: Inter-laboratory comparison in urine with Jaffé, HPLC and enzymatic assays

In consideration of its relatively constant urinary excretion rate, creatinine in urine is a useful biochemical parameter to correct the urinary excretion rate of endogenous and exogenous biomolecules. Assays based on the reaction of creatinine and picric acid first reported by Jaffé in 1886 still belong to the most frequently used laboratory approaches for creatinine measurement in urine. Further analytical methods for creatinine include HPLC-UV, GC-MS, and LC-MS and LC-MS/MS approaches. In the present article we report on the development, validation and biomedical application of a new GC-MS method for the reliable quantitative determination of creatinine in human urine, plasma and serum. This method is based on the derivatization of creatinine (d(0)-Crea) and the internal standard [methyl-trideutero]creatinine (d(3)-Crea) with pentafluorobenzyl (PFB) bromide in the biological sample directly or after dilution with phosphate buffered saline, extraction of the reaction products with toluene and quantification in 1-μl aliquots of the toluene extract by selected-ion monitoring of m/z 112 for d(0)-Crea-PFB and m/z 115 for d(3)-Crea-PFB in the electron-capture negative-ion chemical ionization mode. The limit of detection of the method is 100 amol of creatinine. In an inter-laboratory study on urine samples from 100 healthy subjects, the GC-MS method was used to test the reliability of currently used Jaffé, enzymatic and HPLC assays in clinical and occupational studies. The results of the inter-laboratory study indicate that all three tested methods allow for satisfactory quantification of creatinine in human urine. The GC-MS method is suitable for use as a reference method for urinary creatinine in humans. In serum, creatine was found to contribute to creatinine up to 20% when measured by the present GC-MS method. The application of the GC-MS method can be extended to other biological samples such as saliva.

Modification of Jaffe's kinetic method decreases bilirubin interference: A preliminary report

The negative interference of bilirubin on serum creatinine determined by the kinetic alkaline picrate (Jaffe) reaction is the unresolved problem. Though high performance liquid chromatography and gas chromatography with mass spectroscopy have been proposed to be gold standards for creatinine estimation but they are not readily available in most of the clinical chemistry laboratories due to economic and technical constraints. Most of the present day analyzers use Jaffe's kinetic method without deproteinization. Though enzymatic methods are now routinely used as most accurate method but they are not acceptable due to cost constraints. Hence this study was planned to find out a possible solution to the problem of bilirubin interference by a minor modification in the commonly used Jaffe method so that it is amenable for use on the currently used analyzers.

Effects of sildenafil on maternal hemodynamics and fetal growth in normal rat pregnancy

It has been suggested that the phosphodiesterase-5 (PDE5) inhibitor sildenafil may be useful in the treatment of hypertension during pregnancy. However, we have reported a selective increase in renal inner medullary PDE5 that participates in the sodium retention of pregnancy. Therefore, the purpose of this study was to determine whether oral sildenafil treatment impairs maternal plasma volume expansion and/or fetal growth during rat pregnancy. Rats received sildenafil (10 mg x kg(-1) x day(-1), 50 mg x kg(-1) x day(-1), or 90 mg x kg(-1) x day(-1)) or vehicle on days 4-20 of pregnancy. On days 14-19, rats were housed in metabolic cages for collection of urine and measurement of food and water intake. Terminal hemodynamic and fetal measurements were taken on day 20. None of the sildenafil doses lowered blood pressure, and although all doses increased plasma cGMP concentrations, only the highest dose increased aortic and inner medullary cGMP content. Sildenafil had no effect on maternal weight gain; however, the highest dose decreased both plasma volume and renal sodium retention. The pup number and size were similar among the groups. Therefore, these studies suggest that low doses of systemic sildenafil may be safe during pregnancy in the rat, but higher doses may interfere with the physiological sodium retention and volume expansion of pregnancy. The effects of systemic sildenafil on blood pressure and sodium retention during hypertension in human pregnancy remain to be examined.

Asymmetric dimethylarginine in angiotensin II-induced hypertension

Recent studies have shown that asymmetric dimethylarginine (ADMA), a nitric oxide synthase inhibitor, is increased in hypertension and chronic kidney disease. However, little is known about the effects of hypertension per se on ADMA metabolism. The purpose of this study was to test the hypothesis that ANG II-induced hypertension, in the absence of renal injury, is associated with increased oxidative stress and plasma and renal cortex ADMA levels in rats. Male Sprague-Dawley rats were treated with ANG II at 200 ng.kg(-1).min(-1) sc (by minipump) for 1 or 3 wk or at 400 ng.kg(-1).min(-1) for 6 wk. Mean arterial pressure was increased after 3 and 6 wk of ANG II; however, renal injury (proteinuria, glomerular sclerosis, and interstitial fibrosis) was only evident after 6 wk of treatment. Plasma thiobarbituric acid reactive substances concentration and renal cortex p22(phox) protein abundance were increased early (1 and 3 wk), but urinary excretion of isoprostane and H(2)O(2) was only increased after 6 wk of ANG II. An increased in plasma ADMA after 6 wk of ANG II was associated with increased lung protein arginine methyltransferase-1 abundance and decreased renal cortex dimethylarginine dimethylaminohydrolase activity. No changes in renal cortex ADMA were observed. ANG II hypertension in the absence of renal injury is not associated with increased ADMA; however, when the severity and duration of the treatment were increased, plasma ADMA increased. These data suggest that elevated blood pressure alone, for up to 3 wk, in the absence of renal injury does not play an important role in the regulation of ADMA. However, the presence of renal injury and sustained hypertension for 6 wk increases ADMA levels and contributes to nitric oxide deficiency and cardiovascular disease.

In vivo renal arginine release is impaired throughout development of chronic kidney disease

The kidney is a major site of arginine synthesis where citrulline is converted to arginine via argininosuccinate synthase (ASS) and lyase (ASL). The rate-limiting step in arginine synthesis by the normal kidney is the rate of citrulline delivery and uptake to the renal cortex. We tested whether with chronic kidney disease (CKD) renal arginine synthesis may be compromised. Using the 5/6 renal ablation/infarction (A/I) injury model, we measured renal citrulline delivery and uptake as well as arginine release at early, moderate, and severe stages of CKD vs. healthy controls. The renal plasma flow (RPF) and arterial-renal venous difference was measured at baseline and during citrulline infusion. Citrulline delivery was reduced at all stages of disease due to marked reductions in RPF and despite moderately increased plasma citrulline. Early after 5/6 A/I, the kidney demonstrated a compensatory increase in citrulline uptake while at moderate and severe injury baseline citrulline uptake fell. At all stages of CKD, renal arginine release was markedly reduced. Citrulline infusion increased plasma citrulline in all groups, resulting in increased renal delivery vs. baseline. In healthy kidneys and early injury, citrulline uptake increased with the infusion, but only in the normal kidney did arginine production increase in parallel with the increased citrulline uptake. At moderate and severe injury, there was no increase in citrulline uptake or arginine production. The fall in arginine production in 5/6 A/I was due to an early loss of ASS and ASL conversion of citrulline, which combined with a later reduction in citrulline uptake.

Developmental changes in the L-arginine/nitric oxide pathway from infancy to adulthood: plasma asymmetric dimethylarginine levels decrease with age

BACKGROUND

The L-arginine/nitric oxide (NO) pathway has multiple physiological functions including vasodilation, inhibition of platelet aggregation and neurotransmission. Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of all known NO synthase isoforms, has adverse effects on renal and cardiovascular function in adults. It is unknown whether ADMA might also exert similar effects in younger individuals including infants. Also, reference data for important members of the L-arginine/NO family, notably ADMA and the NO metabolites, nitrite and nitrate, in infancy are lacking.

METHODS

In the present study, we investigated the status of the L-arginine/NO pathway in 34 healthy volunteers aged 2 days to 24 years by measuring the concentration of ADMA, nitrite, nitrate and L-arginine in plasma and urine using gas chromatography-mass spectrometry and gas chromatography-tandem mass spectrometry methods.

RESULTS

We found that ADMA levels in plasma decreased with age (Pearson correlation coefficient r=-0.619, p<0.001). In contrast, urinary excretion of nitrate (r=0.471, p=0.036) and nitrite increased with age (r=0.484, p=0.037).

CONCLUSIONS

Our study suggests that in infants ADMA biosynthesis accompanied by an inhibition of NO synthesis is higher than in adults and diminishes considerably with age.

Creatinine measurements in 24 h urine by liquid chromatography--tandem Mass Spectrometry

A simple, sensitive, and specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for determining urinary creatinine was developed and used to evaluate 24 h urine samples collected during an exposure study. Urine (1 microL) was diluted with methanol and then directly applied to LC-MS/MS. Under electrospray ionization (ESI) conditions, the transition molecules of creatinine and creatinine- d3 were observed at m/ z 114 > 44 and m/ z 117 > 47, respectively. The retention time of creatinine was 0.59 min. The linear range was 1-2000 ng/mL, with a detection limit in urine of 1 ng/mL. LC-MS/MS and colorimetric end-point methods were significantly associated ( R2 = 0.8785, p < 0.0001). The LC-MS/MS method to determine creatinine in 24 h urine samples had shorter retention times, was more sensitive, reliable, reproducible, simple, selective, and used a smaller sample size than other LC-MS/MS or commercial methods.

GC–MS assay for hepatic DDAH activity in diabetic and non-diabetic rats by measuring dimethylamine (DMA) formed from asymmetric dimethylarginine (ADMA): Evaluation of the importance of S-nitrosothiols as inhibitors of DDAH activity in vitro and in vivo in humans

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis, is hydrolyzed to dimethylamine (DMA) and L-citrulline by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). In the present article we report on a GC-MS assay for DDAH activity in rat liver homogenate in phosphate buffered saline. The method is based on the quantitative determination of ADMA-derived DMA by GC-MS as the pentafluorobenzamide derivative. Quantification was performed by selected-ion monitoring of the protonated molecules at m/z 240 for DMA and m/z 246 for the internal standard (CD3)2NH in the positive-ion chemical ionization mode. The assay was applied to determine the enzyme kinetics in rat liver, the hepatic DDAH activity in streptozotocin-induced (50 mg/kg) diabetes in rats, and to evaluate the importance of S-nitrosothiols as DDAH inhibitors. The KM and Vmax values were determined to be 60 microM ADMA and 12.5 pmol DMA/minmg liver corresponding to 166 pmol DMA/minmg protein. Typical DDAH activity values measured in rat liver homogenate were 8.7 pmol DMA/minmg liver at added ADMA concentration of 100 microM. DDAH activity was found to be 1.7-fold elevated in diabetic as compared to non-diabetic rats (P=0.01). The SH-specific agents HgCl2, S-nitrosocysteine ethyl ester (SNACET), a synthetic lipophilic S-nitrosothiol, S-nitrosoglutathione (GSNO), S-nitrosocysteine (CysNO) and S-nitrosohomocysteine (HcysNO) were found to inhibit DDAH activity in rat liver homogenate. The IC50 values for HcysNO, SNACET, CysNO and GSNO were estimated to be 300, 500, 700 and 1000 microM, respectively. Oral administration of 15N-labelled SNACET to two healthy volunteers (1 micromol/kg) resulted in elevated urinary excretion of 15N-labelled nitrite and nitrate, but did not reduce creatinine-corrected excretion of DMA in the urine. Our results suggest that inhibition of DDAH activity on the basis of reversible nitros(yl)ation or irreversible N-thiosulfoximidation of the sulfhydryl group of the cysteine moiety involved in the catalytic process is most likely not a rationale design of DDAH inhibitors. A major advantage of the present GC-MS assay over other assays is that DDAH activity is assessed by measuring the formation of the specific enzymatic product DMA but not the formation of unlabelled or (radio)labelled L-citrulline or the decay of the substrate ADMA. The GC-MS assay reported here should be suitable to probe for DDAH activity in various disease models.

Stromal cells protect against acute tubular injury via an endocrine effect

Emerging evidence suggests that the intravenous injection of bone marrow-derived stromal cells (BMSC) improves renal function after acute tubular injury, but the mechanism of this effect is controversial. In this article, we confirm that intravenous infusion of male BMSC reduced the severity of cisplatin-induced acute renal failure in adult female mice. This effect was also seen when BMSC (or adipocyte-derived stromal cells (AdSC)), were given by intraperitoneal injection. Infusion of BMSC enhanced tubular cell proliferation after injury and decreased tubular cell apoptosis. Using the Y chromosome as a marker of donor stromal cells, examination of multiple kidney sections at one or four days after cell infusion failed to reveal any examples of stromal cells within the tubules, and only rare examples of stromal cells within the renal interstitium. Furthermore, conditioned media from cultured stromal cells induced migration and proliferation of kidney-derived epithelial cells and significantly diminished cisplatin-induced proximal tubule cell death in vitro. Intraperitoneal administration of this conditioned medium to mice injected with cisplatin diminished tubular cell apoptosis, increased survival, and limited renal injury. Thus, marrow stromal cells protect the kidney from toxic injury by secreting factors that limit apoptosis and enhance proliferation of the endogenous tubular cells, suggesting that transplantation of the cells themselves is not necessary. Identification of the stromal cell-derived protective factors may provide new therapeutic options to explore in humans with acute kidney injury.

Plasma creatinine determination in mice and rats: An enzymatic method compares favorably with a high-performance liquid chromatography assay

The use of the colorimetric Jaffé method for the measurement of creatinine in mouse and rat plasma has been criticized as prior studies have shown a dramatic overestimation. We compared a colorimetric picric acid, an enzymatic, and a high-performance liquid chromatography (HPLC) method to assess their appropriateness for routine measurements of creatinine in plasma of healthy and diseased mice (n=61) and rats (n=56). For the colorimetric Jaffé method a pronounced overestimation is confirmed. Additionally the method showed interference with hemoglobin already in a very low, non-visible concentration range in rat plasma. The enzymatic measurement demonstrated a hemoglobin interference in mice, only when hemolysis was visible. The comparison between HPLC and the enzymatic measurement gave a good agreement between both methods in both species. Therefore the enzymatic method fulfills the requirements for a routine screening test for plasma creatinine in healthy as well as diseased mice and rats Kiover a broad concentration range.

Elevated plasma concentrations of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine in citrullinemia

Citrullinemia is an inborn error of the urea cycle with deficiency of the argininosuccinate synthase. It is characterized by elevated concentrations of l-citrulline and decreased levels of l-arginine in body fluids. Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase that converts l-arginine to l-citrulline and nitric oxide (NO). Asymmetric dimethylarginine is hydrolyzed by the enzyme dimethylarginine dimethylaminohydrolase to l-citrulline and dimethylamine. Elevation of l-citrulline in citrullinemia prompted us to study the l-arginine/NO pathway in this disorder. In 8 children with citrullinemia (3 days to 3 years of age), elevated plasma levels of asymmetric dimethylarginine (P = .028) were found compared with age-matched healthy children. We hypothesize that the l-arginine/NO pathway plays a role in the pathophysiology of citrullinemia.

Analysis of creatinine in mouse and rat serum by ion exchange high performance liquid chromatography for in vivo studies of renal function

An ion exchange high performance liquid chromatography method was developed for determining creatinine levels in both mouse and rat serum samples. Separation of creatinine from other serum components was achieved in 10 min using a 100 x 4.1-mm, 10 microm strong cation exchange column following acetonitrile precipitation of serum proteins. Incorporation of a guard cartridge placed in-line prior to the analytical column was employed to prevent interference from compounds used in renal disease animal trials. Creatinine levels in normal and diseased animals were accurately determined in the 0.01-10 mg/dL range, and average recovery of the method was approximately 85% for both mouse and rat serum. Addition of 0.5-1.0% acetic acid to the acetonitrile used for protein precipitation significantly improved creatinine recovery to above 97% in mouse serum. The method was used for routine preclinical diagnosis of rat and mouse model renal function, and for the evaluation of renal disease treatment efficacy.

Accurate quantification of dimethylamine (DMA) in human urine by gas chromatography-mass spectrometry as pentafluorobenzamide derivative: evaluation of the relationship between DMA and its precursor asymmetric dimethylarginine (ADMA) in health and disease

Dimethylamine [DMA, (CH(3))(2)NH)] is abundantly present in human urine. Main sources of urinary DMA have been reported to include trimethylamine N-oxide, a common food component, and asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis. ADMA is excreted in the urine in part unmetabolized and in part after hydrolysis to DMA by dimethylarginine dimethylaminohydrolase (DDAH). Here we describe a GC-MS method for the accurate and rapid quantification of DMA in human urine. The method involves use of (CD(3))(2)NH as internal standard, simultaneous derivatization with pentafluorobenzoyl chloride and extraction in toluene, and selected-ion monitoring of m/z 239 for DMA and m/z 245 for (CD(3))(2)NH in the electron ionization mode. GC-MS analysis of urine samples from 10 healthy volunteers revealed a DMA concentration of 264+/-173 microM equivalent to 10.1+/-1.64 micromol/mmol creatinine. GC-tandem MS analysis of the same urine samples revealed an ADMA concentration of 27.3+/-15.3 microM corresponding to 1.35+/-1.2 micromol/mmol creatinine. In these volunteers, a positive correlation (R=0.83919, P=0.0024) was found between urinary DMA and ADMA, with the DMA/ADMA molar ratio being 10.8+/-6.2. Elevated excretion rates of DMA (52.9+/-18.5 micromol/mmol creatinine) and ADMA (3.85+/-1.65 micromol/mmol creatinine) were found by the method in 49 patients suffering from coronary artery disease, with the DMA/ADMA molar ratio also being elevated (16.8+/-12.8). In 12 patients suffering from end-stage liver disease, excretion rates of DMA (47.8+/-19.7 micromol/mmol creatinine) and ADMA (5.6+/-1.5 micromol/mmol creatinine) were found to be elevated, with the DMA/ADMA molar ratio (9.17+/-4.2) being insignificantly lower (P=0.46). Between urinary DMA and ADMA there was a positive correlation (R=0.6655, P<0.0001) in coronary artery disease, but no correlation (R=0.27339) was found in end-stage liver disease.

Determination of creatinine in urine by tandem mass spectrometry

BACKGROUND

Electrospray tandem mass spectrometry (TMS) is a very powerful tool that enables one to perform high sample throughput analysis. This paper describes a method to determine creatinine in urine by tandem mass spectrometry with direct sample infusion into an ion source.

METHODS

Samples (50 microl) were diluted with internal standard (IS) (450 microl of 0.667 mmol/l deuterated creatinine). Diluted samples were introduced into mass spectrometer with no prior pretreatment and after purification on ion-exchange 96-column cartridge. Tandem mass spectrometry analyses were performed in selected reaction monitoring mode. Creatinine and creatinine-d(3) were monitored using precursor and product ion settings (m/z 114 to 86 and m/z 117 to 89, respectively). The time of an analysis was 3.015 min. Both TMS methods were compared mutually and with the results obtained by enzymatic and Jaffe method.

RESULTS

Linearity was obtained in the range 0.06-60 mmol/l. Detection limit was 0.2 mumol/l and recoveries were in the range 95.1-98.3% for both the assays with and without ion-exchange column. Results of both assays are in good agreement with those obtained by enzymatic and Jaffe method based on log-transformed Bland-Altman plots. Electrospray tandem mass spectrometry method utilizing both approaches with and without ion-exchange column is acceptable according to CLIA criteria.

CONCLUSION

Tandem mass spectrometry allows rapid, sensitive and selective determination of creatinine in untreated urine.