Characterization of immunochemically nonreactive urinary albumin

A New Method for Albuminuria Measurement Using a Specific Reaction between Albumin and the Luciferin of the Firefly Squid Watasenia scintillans

This study demonstrates that the luciferin of the firefly squid Watasenia scintillans, which generally reacts with Watasenia luciferase, reacted with human albumin to emit light in proportion to the albumin concentration. The luminescence showed a peak wavelength at 540 nm and was eliminated by heat or protease treatment. We used urine samples collected from patients with diabetes to quantify urinary albumin concentration, which is essential for the early diagnosis of diabetic nephropathy. Consequently, we were able to measure urinary albumin concentrations by precipitating urinary proteins with acetone before the reaction with luciferin. A correlation was found with the result of the immunoturbidimetric method; however, the Watasenia luciferin method tended to produce lower albumin concentrations. This may be because the Watasenia luciferin reacts with only intact albumin. Therefore, the quantification method using Watasenia luciferin is a new principle of urinary albumin measurement that differs from already established methods such as immunoturbidimetry and high-performance liquid chromatography.

Reconsidering Garth Robinson: fluid flow and the glomerular filtration barrier

PURPOSE OF REVIEW

The goal of this review is to present recent models of the filtration barrier that may suggest mechanism-based treatments for proteinuric renal disease. The vast majority of renal failure occurs in diseases of glomerular proteinuria. The physiology of the filtration barrier remains incompletely understood, preventing invention of mechanism-based therapies. Research is currently dominated by molecular biology approaches to the kidney instead of engineering-based filtration and transport models.

RECENT FINDINGS

Reexamination of two older paradigms (basement membrane and slit diaphragm) and critical analysis of newer models may provide mechanistic insight to guide further research. We expand on our theory of podocyte-basement membrane mechanical interactions and speculate on mechanisms of action of the leading treatment for proteinuria, angiotensin blockade.

SUMMARY

Treatment of proteinuria remains largely empiric and based on inhibition of the renin-angiotensin-aldosterone system, with additional benefit from statins and vitamin D. Improved definition of transport phenomena in the capillary wall may suggest rational design of new interventions.

A cost-effective method for purification and characterization of human urinary albumin

We describe a simplified approach for the purification and characterization of urinary albumin, a key biomarker currently used for understanding the onset and prognosis of microalbuminuria. Urinary albumin was purified from human urine collected from diabetic kidney disease patients by using 2-stage tangential flow filtration process and set of column chromatography steps. The relative molecular mass of urinary albumin is 66,871 Da (SYNAPT G2 High Definition Mass Spectrometry System). Isolated urinary albumin was analyzed by SDS-PAGE, Western blotting, immunoelectrophoresis, Ouchterlony double-immunodiffusion, single radial immunodiffusion, size-exclusion HPLC and peptide mass fingerprint analysis. The size-exclusion HPLC elution profile of the purified urinary albumin was similar to that of a reference form of native albumin. Peptide mass fingerprint analysis of the purified urinary albumin yielded peptides that partially matched with known sequence of ALBU_HUMAN (P02768). This is the first report of purification and validation of immunochemically reactive form of urinary albumin from a large volume of urine of diabetic kidney disease patients. In this purification approach, the cost of the purified albumin is significantly lower.

Methods of albumin estimation in clinical biochemistry: Past, present, and future

Estimation of serum and urinary albumin is routinely performed in clinical biochemistry laboratories. In the past, precipitation-based methods were popular for estimation of human serum albumin (HSA). Currently, dye-binding or immunochemical methods are widely practiced. Each of these methods has its limitations. Research endeavors to overcome such limitations are on-going. The current trends in methodological aspects of albumin estimation guiding the field have not been reviewed. Therefore, it is the need of the hour to review several aspects of albumin estimation. The present review focuses on the modern trends of research from a conceptual point of view and gives an overview of recent developments to offer the readers a comprehensive understanding of the subject.

Determination of Urine Albumin by New Simple High-Performance Liquid Chromatography Method

BACKGROUND

A simple high-performance liquid chromatography (HPLC) method was developed for the determination of albumin in patients' urine samples without coeluting proteins and was compared with the immunoturbidimetric determination of albumin. Urine albumin is important biomarker in diabetic patients, but part of it is immuno-nonreactive.

METHODS

Albumin was determined by high-performance liquid chromatography (HPLC), UV detection at 280 nm, Zorbax 300SB-C3 column. Immunoturbidimetric analysis was performed using commercial kit on automatic biochemistry analyzer COBAS INTEGRA^® 400, Roche Diagnostics GmbH, Manheim, Germany.

RESULTS

The HLPC method was fully validated. No significant interference with other proteins (transferrin, α-1-acid glycoprotein, α-1-antichymotrypsin, antitrypsin, hemopexin) was found. The results from 301 urine samples were compared with immunochemical determination. We found a statistically significant difference between these methods (P = 0.0001, Mann-Whitney test).

CONCLUSION

New simple HPLC method was developed for the determination of urine albumin without coeluting proteins. Our data indicate that the HPLC method is highly specific and more sensitive than immunoturbidimetry.

Urinary Albumin Detection: Comparison of Two Different Methods

BACKGROUND

Monitoring urinary albumin is a useful method in clinical practice for the management of diabetic nephropathy, chronic kidney disease, and hypertension. Currently there are neither standardized methods nor reference material for the determination of urinary albumin; for this reason it is useful to compare different assays used in clinical laboratory.

OBJECTIVES

The aim of this study is to verify analytical performance of an immunoturbidimetric assay on Roche Cobas 8000 platform and to compare urinary albumin results with those obtained by immunonephelometry on Siemens Dade Behring BN II Nephelometer.

RESULTS

The method comparison showed a good linear relationship, confirmed by Passing-Bablok and Bland-Altman plots. The turbidimetric assay meets the requirements of accuracy and precision for the practice of medical diagnostics and clinical use.

CONCLUSIONS

The present study can contribute to the methods standardization and harmonization of urinary albumin assay.

Investigation of apparent non-albuminuric proteinuria in a primary care population

BACKGROUND

There is debate as to whether using the urinary albumin- or protein-to-creatinine ratio (ACR or PCR) should be the primary test for proteinuria. Whilst albuminuria (increased ACR) in the absence of proteinuria (increased PCR) may be expected in some patients, the converse (i.e., proteinuria in the absence of albuminuria) is more unusual and its cause and significance are unclear. We investigated the nature of such apparent non-albuminuric proteinuria in a primary care population of patients.

METHODS

ACR and PCR were measured in 569 urine samples from patients who either had chronic kidney disease or were at increased risk of the condition. Samples with apparent proteinuria (PCR ≥23 mg/mmol/≥200 mg/g) but no albuminuria (ACR <3.4 mg/mmol/<30 mg/g) were classified as 'discrepant' (37% of proteinuric samples, 6% of all samples); 27 of these samples were available for further analyses. The further analyses included electrophoresis, repeat measurement, immunoassays for markers of tubular proteinuria and use of alternative albumin and total protein methods.

RESULTS

Electrophoresis did not identify significant proteinuria in the discrepant samples. The only evidence of tubular proteinuria following measurement of three urinary markers of the condition was a mildly increased α1-microglobulin-to-creatinine ratio in 10 of the 27 discrepant samples analysed, four of which also had a raised β-trace protein-to-creatinine ratio. Use of an alternative urinary total protein method resulted in significantly lower PCRs and 17 of the 27 samples were no longer classified as proteinuric.

CONCLUSIONS

We were unable to confirm the cause of a raised PCR without albuminuria in these patients and suspect that in most cases it is artefactual.

Immunochemically unreactive albumin in urine: fiction or reality?

Urinary albumin measurements are currently not standardized due to a lack of a reference method and reference (primary and secondary [matrix]) material. Multiple molecular forms of albumin in urine are identified. Modification of albumin by proteolysis during passage through the urinary tract and chemical modification during specimen storage leads to the formation of albumin fragments. Multiple methods have been developed to quantify albuminuria and significant different results are reported dependent on the available assay. The current point of view of the National Kidney Disease Education Program - IFCC Working Group on Standardization of Albumin considers the immunoassay with polyclonal sera as the primary method of quantifying urine albumin. This article reviews the process of albumin fragmentation and focuses on the controversial topic of immuno-unreactive, nonimmunoreactive, or immunochemically nonreactive albumin fractions and its consequences for albumin analysis. We conclude that at present there are no hard arguments for measuring immunochemically unreactive albumin in urine. Immunoassays using polyclonal antisera for the detection of urinary albumin remain the gold standard. The development of a reference measurement procedure remains one of the challenges for the future.

Assessment of proteinuria

Proteinuria is a strong predictor of adverse cardiovascular and kidney events, and an accurate assessment of proteinuria is important for the evaluation and management of CKD. Total urinary protein can be assessed using dipstick, precipitation, and electrophoresis methods. Urinary albumin, the predominant urinary protein in most proteinuric kidney diseases, can be assessed using an albumin-specific dipstick, immunochemical techniques, and size-exclusion high-performance liquid chromatography. Urine albumin may be immune-reactive, immune-unreactive, fragmented, and biochemically modified, and laboratory techniques have variable abilities to detect different types of albumin. Urine specimen for proteinuria assessment can either be obtained from a timed-collection or a spot urine sample. Spot urine protein- or albumin-to-creatinine ratios are preferred to a 24-hour urine sample in routine practice. Assessment of albuminuria rather than proteinuria is more clinically meaningful in patients with diabetic kidney disease, and proteinuria and albuminuria assessments both have a role in nondiabetic kidney disease and in general population screening. As measurement and sampling procedures for proteinuria assessment have yet not been standardized, it is important for physicians to be aware of different types of urinary proteins, albumins, laboratory techniques, and urine sampling methods.

Potential urinary biomarkers of disease activity in Crohn's disease

OBJECTIVE

Measurement of the immunoreactive urinary albumin (ir-uAlb) concentration by immunological methods was found to be an effective method to identify disease activity in Crohn's disease (CD). Recently a size-exclusion (SE) high performance liquid chromatography (HPLC) method was developed to measure both ir-uAlb and non-immunoreactive urinary albumin (total, t-uAlb). We aimed to follow-up one of our CD patients with frequent remissions and exacerbation phases comparing the changes of disease activity parameters and the concentration of ir-uAlb and t-uAlb. The surprising results led us to perform measurements in greater depth.

MATERIAL AND METHODS

Concentration of ir-uAlb was measured by immunoturbidimetry (IT) and t-uAlb by SE-HPLC. Albumin peak of SE-HPLC was collected and applied to a reversed-phase (RP) HPLC and to gel-electrophoresis. Eluted peaks of RP-HPLC and identified bands of gel-electrophoresis were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS).

RESULTS

The concentration of t-uAlb was 15 times higher than that of the ir-uAlb during active state. The RP-HPLC and the gel-electrophoresis separation proved that albumin peak by size-exclusion consists of three different peaks. MALDI-TOF/MS measurements identified α1-acid-glycoprotein and Zn-α2-glycoprotein as major, and albumin as minor protein.

CONCLUSIONS

Peak of albumin of SE-HPLC contains a significant amount of glycoprotein during the active phase of CD, which could not be detected in remission. Urinary α1-acid-glycoprotein and/or Zn-α2-glycoprotein could be an ideal disease activity biomarker of CD.

Microalbuminuria, indicated by total versus immunoreactive urinary albumins, in acute ischemic stroke patients

Microalbuminuria, assessed by measuring immunoreactive albumin, is common in patients with cerebrovascular disease and is associated with increased risk of stroke. Total urinary albumin (t-uAlb) comprises both immunoreactive albumin (ir-uAlb) and nonimmunoreactive albumin (nir-uAlb). We hypothesized that t-uAlb is a more sensitive indicator of microalbuminuria than ir-uAlb, and that measurement of t-uAlb will increase the prevalence of microalbuminuria in ischemic stroke patients compared with measurement of ir-uAlb and will show a stronger correlation with the severity of stroke and oxidative stress. In urine samples from 98 patients with ischemic stroke, the albumin-to-creatinine ratios t-uAlb/uCreat and ir-uAlb/uCreat were measured by high-performance liquid chromatography (HPLC) and immunoturbidimetry (IT), and the nir-uAlb/uCreat ratio was calculated. Urinary ortho-tyrosine (o-Tyr/uCreat), an indicator of oxidative stress, was measured by HPLC. The severity of stroke was scored based on the National Institutes of Health Stroke Scale (NIHSS). The prevalence of microalbuminuria detected by HPLC was significantly higher than that detetcted by IT (66.3 vs 36.7%). Although all forms of albumin showed significant correlation with stroke severity (t-uAlb: r = 0.24, P < .05 ir-uAlb: r = 0.25, P < .05 nir-uAlb: r = 0.29, P < .05), only nir-uAlb was found to be an independent predictor of stroke severity (B = 0.20, β = 0.35, P < .05). In addition, t-uAlb/uCreat and nir-uAlb/uCreat had a significant correlation with o-Tyr/uCreat (r = 0.336, P < .05 and r = 0.358, P < .05 respectively), whereas ir-uAlb/uCreat did not (r = 0.22, P > .05). Our data suggest that in acute ischemic stroke patients, t-uAlb is a more sensitive indicator of microalbuminuria than the presently used ir-uAlb. Future studies should aim to elucidate the underlying mechanisms for the relationship among urinary albumins and cerebrovascular diseases and the role of urinary albumins in risk stratification for stroke.

Interpreting abnormal proteinuria in pregnancy: the need for a more pathophysiological approach

This review and opinion article focuses on the definitions and meanings of abnormal protein excretion in pregnancy, asking the following questions: Are our tests to determine abnormal proteinuria adequately performed? Are current guidelines for diagnosis of excessive proteinuria, especially when used to identify preeclampsia, supported by adequate data? Can the magnitude of proteinuria be used as a reliable clinical biomarker of the gravity of preeclampsia? Should timed urine collections, primarily 24-hour excretions, be supplanted by the urine protein/creatinine ratio in clinical practice? The answers to most of these questions are: We are not sure, or some guidelines are poorly supported by data and may prove erroneous. We suggest a more physiologic approach to assessment of proteinuria and believe that if clinicians and investigators looked at proteinuria in terms of how the kidney handles protein in health and disease it would lead to a more rational and evidence-based approach to proteinuria in pregnancy. Finally, we recommend that current cutoff for abnormal proteinuria be used to diagnose preeclampsia, but the level of proteinuria should not guide management. Other variables, such as status of blood pressure control, evidence of increasing organ damage in the liver and hematological systems, evidence of falling glomerular filtration rate, and signs of neurological involvement, are more reliable indicators of severity of preeclampsia.

Urinary proteases degrade albumin: implications for measurement of albuminuria in stored samples

BACKGROUND

Previous studies have shown that albumin in stored urine samples degrades over time, and that albumin losses are greatest in samples with low pH conditions (pH < 5). Furthermore, the high-performance liquid chromatography (HPLC) assay for urinary albumin has been shown to be particularly susceptible to the effects of prolonged storage.

METHODS

Frozen urine samples, stored for 12 months at -70 and -20 degrees C, were analysed for albumin fragmentation. Urinary protease activity was investigated in vitro in urine adjusted to pH 2.3-2.5. Albumin was measured by nephelometry, HPLC and sodium dodecyl sulphate-polyacrylamide gel electrophoresis.

RESULTS

In the unadjusted samples, albumin was degraded in 11 out of 40 samples stored at -20 degrees C. In the in vitro experiments, both endogenous albumin and exogenous albumin added to urine were rapidly degraded into large fragments within minutes after adjustment to low pH. The fragments produced were consistent with those produced during digestion with pepsin and urinary degradation was completely inhibited by pepstatin. Albumin concentration measured by HPLC was most dramatically affected, with near-complete loss of albumin-sized material within one hour of incubation at pH 2.3-2.5. Sample reactivity with antiserum in a nephelometry assay initially declined then increased, possibly due to exposure of internal epitopes during albumin digestion.

CONCLUSIONS

This study demonstrated that proteases are present and active in stored human urine samples. Urinary albumin digestion occurred in a manner consistent with activity of endogenous urinary proteases. Adjustment to neutral pH or addition of protease inhibitors may be useful techniques for sample preservation.

PROTEINURIA IN PREECLAMPSIA: FROM BENCH TO BEDSIDE

Pre-eclampsia (PE) remains the leading cause of maternal and fetal mortality in the developed world and parts of the developing world. Morbidity and mortality from PE is increased in the developing world compared to the developed world, as availability and access to antenatal care and pathology services are limited.

How should proteinuria be detected and measured?

Proteinuria is a classic sign of kidney disease and its presence carries powerful prognostic information. Although proteinuria testing is enshrined in clinical practice guidelines, there is surprising variation among such guidelines as to the definition of clinically significant proteinuria. There is also poor agreement as to whether proteinuria should be defined in terms of albumin or total protein loss, with a different approach being used to stratify diabetic and non-diabetic nephropathy. Further, the role of reagent strip devices in the detection and assessment of proteinuria is unclear. This review explores these issues in relation to recent national and international guidelines on chronic kidney disease (CKD) and epidemiological evidence linking proteinuria and clinical outcome. The authors argue that use of urinary albumin measurement as the front-line test for proteinuria detection offers the best chance of improving the sensitivity, quality and consistency of approach to the early detection and management of CKD.

Early detection of microalbuminuria and hypertension in children of very low birthweight

OBJECTIVE

We retrospectively investigated a cohort of very low birthweight infants (VLBWI) by comparing two methods for assessment of albuminuria: nephelometry (standard) and high performance liquid chromatography (HPLC), measuring urinary immunoreactive and non-immunoreactive albumin to evaluate if the latter is more sensitive in identifying childhood onset of albuminuria and hypertension in VLBWI individuals.

METHODS

Spot urine samples of a total of 109 subjects (5.3 +/- 2.2 years old) who were VLBWI were investigated by HPLC and nephelometry. Twenty-eight subjects were small for gestational age (SGA) and 81 were appropriate for gestational age (AGA). Blood Pressure (BP) was also recorded.

RESULTS

Twelve children (11%) with albuminuria 20 mg/g Cr by nephelometry versus 48 (44.5%) by HPLC (p < 0.001) were identified. The mean +/- SD of values in the range of normoalbuminuria was 2.9 +/- 5.81 by nephelometry versus 22.6 +/- 14 for HPLC (p < 0.001). The values were not statistically different in the two birthweight categories (<1000, 1000-1499 g), in AGA versus SGA and in high BP (>90th percentile) versus normal BP children.

CONCLUSIONS

Microalbuminuria by HPLC was more marked in VLBWI than in adults and the difference between values using the two methods was significative.

Albuminuria: what can we expect from the determination of nonimmunoreactive albumin?

Albuminuria is an early marker for diabetic nephropathy in patients with diabetes, and has a clear place in patient care. It also predicts cardiovascular events and mortality in diabetic patients and in the general population, and is slowly becoming accepted in population screening for cardiovascular disease and chronic kidney disease. Recently, investigators found that a considerable amount of albumin in urine is nonimmunoreactive and that classic immunochemical assays do not properly measure all albumin in urine. Assays that detect immunoreactive plus nonimmunoreactive albumin may better predict development of diabetic nephropathy, cardiovascular events, and mortality than assays that only detect immunoreactive albumin. Proof of the existence of nonimmunoreactive albumin emerged from the finding that albumin contains urine fragments. In this review, we critically appraise the presence and relevance of albumin fragments and nonimmunoreactive albumin molecules in urine, and the potential additive value of albuminuria detected by assays that assess nonimmunoreactive plus immunoreactive albumin over albuminuria detected by classic immunochemical assays in predicting end points.

Quantification of urinary albumin by using protein cleavage and LC-MS/MS

BACKGROUND

Urinary albumin excretion is a sensitive diagnostic and prognostic marker for renal disease. Therefore, measurement of urinary albumin must be accurate and precise. We have developed a method to quantify intact urinary albumin with a low limit of quantification (LOQ).

METHODS

We constructed an external calibration curve using purified human serum albumin (HSA) added to a charcoal-stripped urine matrix. We then added an internal standard, (15)N-labeled recombinant HSA ((15)NrHSA), to the calibrators, controls, and patient urine samples. The samples were reduced, alkylated, and digested with trypsin. The concentration of albumin in each sample was determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and linear regression analysis, in which the relative abundance area ratio of the tryptic peptides (42)LVNEVTEFAK(51) and (526)QTALVELVK(534) from albumin and (15)NrHSA were referenced to the calibration curve.

RESULTS

The lower limit of quantification was 3.13 mg/L, and the linear dynamic range was 3.13-200 mg/L. Replicate digests from low, medium, and high controls (n = 5) gave intraassay imprecision CVs of 2.8%-11.0% for the peptide (42)LVNEVTEFAK(51), and 1.9%-12.3% for the (526)QTALVELVK(534) peptide. Interassay imprecision of the controls for a period of 10 consecutive days (n = 10) yielded CVs of 1.5%-14.8% for the (42)LVNEVTEFAK(51) peptide, and 6.4%-14.1% for the (526)QTALVELVK(534) peptide. For the (42)LVNEVTEFAK(51) peptide, a method comparison between LC-MS/MS and an immunoturbidometric method for 138 patient samples gave an R(2) value of 0.97 and a regression line of y = 0.99x + 23.16.

CONCLUSIONS

Urinary albumin can be quantified by a protein cleavage LC-MS/MS method using a (15)NrHSA internal standard. This method provides improved analytical performance in the clinically relevant range compared to a commercially available immunoturbidometric assay.

Chip Electrophoresis as a Method for Quantifying Total Albumin in Cerebrospinal Fluid

The cerebrospinal/serum albumin ratio ( QAlb) is a measure of cerebrospinal—blood barrier permeability, where dysfunctional barriers have been associated with a variety of disease states. Thus, the ability to accurately quantify albumin has critical diagnostic importance. The current work uses chip electrophoresis as an alternative methodology to the conventional immunoassay for quantifying cerebrospinal fluid (CSF) albumin. With variable concentrations of bovine serum albumin normalized to a chicken albumin internal standard, the electrophoresis system demonstrated signal correlation with a best-fit polynomial ( R2 = 0.9993) (concentration range of 7.8–1000 mg/L). Interchip and intrachip variation studies conducted on patient CSF demonstrated coefficient of variations of 0.1–17%. Chip electrophoresis detected an average of 24% greater albumin than the immunoassay in patient CSF samples (n = 58), indicating the ability to detect immunoreactive and non-immunoreactive forms of albumin. Finally, the correlation of CSF albumin concentration to QAlb determined by chip electrophoresis ( r2 = 0.8683) was comparable with that determined by immunoassay ( r2 = 0.8390). Therefore, this work demonstrates proof-of-principle that chip electrophoresis can serve as an alternative method for quantifying total (immunoreactive and non-immunoreactive) albumin in CSF.

Proteinuria without albuminuria: urinary protein excretion by a subset of patients with burn injuries

BACKGROUND

There is disagreement regarding the utility of urinary albumin excretion as a marker for capillary injury in patients with severe burn injuries. We examined protein components in urine specimens from patients with burn injury.

METHODS

Detailed analysis was performed for a set of 5 urine specimens selected based on a high ratio of albumin-sized molecules by size-exclusion HPLC (Accumin) versus albumin by immunoassay methods. Specimens were analyzed for total protein, alpha(1)-microglobulin, alpha(1)-acid glycoprotein, cystatin C, and retinol-binding protein. Urine components were analyzed by chromatographic and electrophoretic methods. Major components were identified by mass spectrometry of tryptic peptides.

RESULTS

A subset of urine specimens had increased total protein with slight increases in albumin by immunoassay or by polyacrylamide gel electrophoresis. Albumin values by size-exclusion HPLC were more than 10-fold higher. Immunoassays for alpha(1)-microglobulin and alpha(1)-acid glycoprotein yielded concentrations 5-10 fold higher than for albumin. Other major components identified included zinc-alpha(2)-glycoprotein and leucine-rich-alpha(2)-glycoprotein.

CONCLUSIONS

A subset of patients with burn injury had increased total urinary protein resulting primarily from increased excretion of proteins such as alpha(1)-microglobulin and alpha(1)-acid glycoprotein with little increase in albumin excretion. The unusual composition of urinary proteins in these patients may relate to decreased filtered load of albumin and increased filtered load of acute phase reactants or to alterations in renal tubular protein processing. Thus, measurement of urinary albumin may have decreased sensitivity for detecting kidney injury in burn patients.

Hermann Senator and albuminuria--forgotten pioneering work in the 19th century

Testing urinary albumin concentration by immune detection methods has recently turned out to be a highly rewarding procedure, as low level albumin excretion has turned out to be a powerful predictor of cardiovascular and renal risk in diabetic and nondiabetic patients. In the following we discuss a text dating back to the 19th century in order to make today's nephrologists aware of the remarkable and prescient, but meanwhile completely forgotten investigations on urinary albumin excretion in individuals without primary kidney disease. The treatise of Hermann Senator convincingly disproved the then held dogma that albuminuria was always a sign of primary renal disease. These observations are all the more remarkable since he was forced to use relatively simple and not absolutely specific methods. He further provided an explanation of the renal handling of albumin which to a large extent is still valid today.

Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to rapidly screen for albuminuria

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is used as an alternative method for the rapid diagnosis of albuminuria. This technique requires no further sample pretreatment than simply mixing the urine sample with a MALDI matrix and drying under ambient conditions. The resulting MALDI mass spectra reveal albumin ions having charges ranging from +1 to +5. The detection of albumin is possible using any of the three most common MALDI matrices - sinapinic acid (SA), 2,5-dihydroxybenzoic acid (2,5-DHB), or 4-hydroxy-alpha-cyanocinnamic acid (alpha-CHC). Using this analytical approach, the limit of detection for albumin in urine is 10(-6) M, approximately 5 to 10 times lower than that detectable through conventional chemical testing.

Current issues in measurement and reporting of urinary albumin excretion

BACKGROUND

Urinary excretion of albumin indicates kidney damage and is recognized as a risk factor for progression of kidney disease and cardiovascular disease. The role of urinary albumin measurements has focused attention on the clinical need for accurate and clearly reported results. The National Kidney Disease Education Program and the IFCC convened a conference to assess the current state of preanalytical, analytical, and postanalytical issues affecting urine albumin measurements and to identify areas needing improvement.

CONTENT

The chemistry of albumin in urine is incompletely understood. Current guidelines recommend the use of the albumin/creatinine ratio (ACR) as a surrogate for the error-prone collection of timed urine samples. Although ACR results are affected by patient preparation and time of day of sample collection, neither is standardized. Considerable intermethod differences have been reported for both albumin and creatinine measurement, but trueness is unknown because there are no reference measurement procedures for albumin and no reference materials for either analyte in urine. The recommended reference intervals for the ACR do not take into account the large intergroup differences in creatinine excretion (e.g., related to differences in age, sex, and ethnicity) nor the continuous increase in risk related to albumin excretion.

DISCUSSION

Clinical needs have been identified for standardization of (a) urine collection methods, (b) urine albumin and creatinine measurements based on a complete reference system, (c) reporting of test results, and (d) reference intervals for the ACR.

Comparison between immunoturbidimetry, size-exclusion chromatography, and LC-MS to quantify urinary albumin

BACKGROUND

The accurate and precise measurement of urinary albumin is critical, since even minor increases are diagnostically sensitive indicators of renal disease, cardiovascular events, and risk for death. To gain insights into potential measurement biases, we systematically compared urine albumin measurements performed by LC-MS, a clinically available immunoturbidimetric assay, and size-exclusion HPLC.

METHODS

We obtained unused clinical urine samples from 150 patients who were stratified by degrees of albuminuria (<20 mg/L, 20-250 mg/L, >250 mg/L) as determined by the immunoturbidimetric assay used in our clinical laboratory (Roche Hitachi 912). Urine albumin was then remeasured via LC-MS and HPLC (Accumin) assays.

RESULTS

The immunoturbidimetric assay, calibrated using manufacturer-supplied serum-derived calibrators (Diasorin), underestimated albumin compared with LC-MS. After calibration with purified HSA, this immunoturbidimetric assay correlated well with LC-MS. HPLC overestimated albumin compared with both LC-MS and immunoturbidimetry. The current LC-MS and HPLC assays both performed poorly at concentrations <20 mg/L.

CONCLUSIONS

Efforts are needed to establish gold-standard traceable calibrators for clinical assays. LC-MS is a specific method to quantify albumin in native urine when concentrations exceed 20 mg/L, and therefore could be employed for standardization among assays.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

Analysis of microalbuminuria with immunonephelometry and high performance liquid chromatography. Evaluation of new criteria

Mind a hipertónia, mind a 2-es típusú diabetes mellitus jelentős tényező a népesség halálozásában. Mindkét betegség károsítja az endothelt, aminek korai jele a microalbuminuria, amelyet szűrővizsgálatként tesztcsíkkal, diagnosztikus vizsgálatként immunológiai alapú módszerekkel, illetve nagy teljesítményű folyadékkromatográfiával lehet mérni. Ez utóbbi segítségével az ún. nem immunreaktív albuminforma is kimutatható. Célkitűzés: A szerzők célkitűzése immunnefelometriával microalbuminuriára negatív, diabéteszes és hipertóniás, illetve nem diabéteszes hipertóniás betegek albuminürítésének vizsgálata volt nagy teljesítményű folyadékkromatográfiával. Továbbá célul tűzték ki a microalbuminuria megállapításához használt jelenlegi kritériumok (albumin-kreatinin hányados: férfiaknál ≥2,5 mg/mmol, nőknél ≥3,5 mg/mmol) és a közelmúltban megjelent Heart Outcomes Prevention Evaluation tanulmány által javasolt új kritériumok (nem diabéteszesekben, immunológiai módszerrel ≥0,7 mg/mmol, nagy teljesítményű folyadékkromatográfiás módszerrel ≥3,1 mg/mmol, diabéteszesekben immunológiai módszerrel ≥1,4 mg/mmol, nagy teljesítményű folyadékkromatográfiával ≥5,2 mg/mmol) használhatóságának vizsgálatát is. Módszer: Szűrővizsgálattal microalbuminuriára negatív 469 egyén vizeletének vizsgálata történt meg immunnefelometriás módszerrel. Az így is negatívakat vizsgálták tovább nagy teljesítményű folyadékkromatográfián alapuló, a méretkizárásos kromatográfia elvén működő Accumin™ Kit-tel. Eredmények: Nagy teljesítményű folyadékkromatográfiával átlagosan háromszor nagyobb albuminürítést mértek, mint immunnefelometriával. Az intraindividuális variációs koefficiens a két módszerrel nem különbözött (37 ± 31% és 40 ± 31%, p = 0,869; immunnefelometria és nagy teljesítményű folyadékkromatográfia; átlag ± szórás). A jelenlegi albumin-kreatinin hányadoson alapuló kritériumokat használva, az immunológiai módszerrel negatív egyének nagy teljesítményű folyadékkromatográfiával 43%-ban bizonyultak pozitívnak. Ha a Heart Outcomes Prevention Evaluation tanulmány új kritériumait használták, 14,5%-ra csökkent az immunnefelometriával negatív, nagy teljesítményű folyadékkromatográfiával pozitívak aránya; nagy teljesítményű folyadékkromatográfiával microalbuminuria-pozitívak száma elsősorban a diabéteszes és hipertóniás csoportban csökkent (49% és 7,5%), míg a nem diabéteszes hipertóniás csoportban kevésbé (37% és 26,5%). A hagyományos kritériumrendszerben logisztikus regressziós vizsgálat során a legerősebb kockázati faktornak a férfinem bizonyult. Az immunnefelometriával microalbuminuriára negatív egyének 28%-ában nagy teljesítményű folyadékkromatográfiával kimondható a microalbuminuria diagnózisa a jelen szakmai szabályok szerint. Következtetések: Az immunológiai módszerekkel microalbuminuriára negatív egyének közel harmadában nagy teljesítményű folyadékkromatográfiával kimondható a microalbuminuria diagnózisa, amihez továbbra is szükséges a háromszori vizeletvizsgálat. A Heart Outcomes Prevention Evaluation tanulmány által megállapított új kritériumok sem a diabéteszes és hipertóniás betegekre, sem a nem diabéteszes hipertóniás betegekre nem alkalmazhatók jól. Nem lehet figyelmen kívül hagyni a microalbuminuria-pozitivitás legjelentősebb prediktorát, a nemet.

Reactivity of Urinary Albumin (Microalbumin) Assays with Fragmented or Modified Albumin

Background: Controversy exists regarding occurrence and measurement of structural variants of albumin in urine. In this study, we examined cross-reactivity of in vitro modified albumins in assays for urine albumin (microalbumin).

Methods: We analyzed albumin modified by reagents, trypsin, or physical treatments or differing in primary sequence (animal albumins) with an immunoturbidimetric assay (Beckman LX20) using goat antiserum and a competitive immunoassay (Siemens Immulite) using a monoclonal antibody. We assessed occurrence of albumin fragments in urine by use of Western blotting of 24 specimens.

Results: Chemical modification, modest sequence substitution (gorilla albumin), or cleavage of albumin by cyanogen bromide (CNBr) had little effect on reactivity in the LX20 assay. Albumin extensively cleaved with trypsin retained partial reactivity. The Immulite assay generally was affected more severely by albumin modifications and sequence changes. Western blots of fresh urine specimens or specimens stored at −80 °C showed little albumin fragmentation, but some specimens stored for 3 years at −20 °C had extensively fragmented albumin that was detected by the LX20 but not the Immulite assay.

Conclusions: Nearly equivalent reactivity of intact albumin and CNBr fragments in the immunoturbidimetric assay indicates reactivity of antibodies with multiple epitopes throughout albumin. Therefore, it is difficult to abolish reactivity of albumin in this type of urine albumin assay. Differential sensitivity of 2 assays to albumin modification identifies a potential source of assay nonequivalence in measuring urinary albumin, particularly for specimens stored at −20 °C.

Respective predictive role of urinary albumin excretion and nonalbumin proteinuria on graft loss and death in renal transplant recipients

Proteinuria is constituted by urinary albumin (UAE) and nonalbumin proteins (NAP). UAE was shown to predict ESRD and death. Whether NAP predicts graft or patient outcome is unknown in renal transplantation. We retrospectively analyzed the impact of UAE and NAP respectively on end-stage renal disease (ESRD) and death in 616 renal transplant recipients. In subjects with proteinuria <0.25 g/day, 76% of urine proteins were NAP; in those with >1 g/day, 44% of the urine proteins were NAP. Determinants of UAE and NAP were partly different: fasting glucose, body weight, donor cause of death and cyclosporine were significantly associated with NAP (but not UAE); panel reactive antibodies (PRA) and rapamycine were significantly associated with UAE (but not with NAP). NAP expressed as a continuous (HR: per g/day: 4.00 [2.85-5.63], p < 0.0001) or a categorical (presence vs. absence, HR = 29.09[8.80-96.20], p < 0.0001) parameter and UAE (per g/day, HR = 1.86 [1.24-2.78], p < 0.0001) were risk factors for graft loss in univariate analyses. NAP remained significant even after adjustment on UAE. The presence of NAP (HR: 5.37 [2.55-11.34], p < 0.0001) and macroalbuminuria (HR: 4.12 [1.65-10.29], p = 0.0024) were risk factors for death. Proteinuria is made of various proportions of UAE and NAP in renal transplantation; these two parameters provide different information on graft/patient survival.

Efficient generation of human hepatocytes by the intrahepatic delivery of clonal human mesenchymal stem cells in fetal sheep

Alternative methods to whole liver transplantation require a suitable cell that can be expanded to obtain sufficient numbers required for successful transplantation while maintaining the ability to differentiate into hepatocytes. Mesenchymal stem cells (MSCs) possess several advantageous characteristics for cell-based therapy and have been shown to be able to differentiate into hepatocytes. Thus, we investigated whether the intrahepatic delivery of human MSCs is a safe and effective method for generating human hepatocytes and whether the route of administration influences the levels of donor-derived hepatocytes and their pattern of distribution throughout the parenchyma of the recipient's liver. Human clonally derived MSCs were transplanted by an intraperitoneal (n = 6) or intrahepatic (n = 6) route into preimmune fetal sheep. The animals were analyzed 56-70 days after transplantation by immunohistochemistry, enzyme-linked immunosorbent assay, and flow cytometry. The intrahepatic injection of human MSCs was safe and resulted in more efficient generation of hepatocytes (12.5% +/- 3.5% versus 2.6% +/- 0.4%). The animals that received an intrahepatic injection exhibited a widespread distribution of hepatocytes throughout the liver parenchyma, whereas an intraperitoneal injection resulted in a preferential periportal distribution of human hepatocytes that produced higher amounts of albumin. Furthermore, hepatocytes were generated from MSCs without the need to first migrate/lodge to the bone marrow and give rise to hematopoietic cells.

CONCLUSION

Our studies provide evidence that MSCs are a valuable source of cells for liver repair and regeneration and that, by the alteration of the site of injection, the generation of hepatocytes occurs in different hepatic zones, suggesting that a combined transplantation approach may be necessary to successfully repopulate the liver with these cells.

HPLC-detected albuminuria predicts mortality

Albuminuria is a risk factor for all-cause mortality. Urinary albumin is traditionally measured with an immunochemical method such as immunonephelometry (IN). High-performance liquid chromatography (HPLC) detects additional albumin that is missed by IN, but it is unknown if HPLC-detected albuminuria also predicts mortality in the general population. We measured urinary albumin at baseline with both IN and HPLC in 10,175 participants in the longitudinal Australian Diabetes, Obesity, and Lifestyle (AusDiab) study. Over 5.2 yr of follow-up, 319 participants died. In a Cox proportional hazards regression model, participants with albuminuria measured by IN or HPLC were approximately twice as likely to die as participants who were normoalbuminuric on both assays. Receiver-operator characteristic curve analyses suggest that each test has a similar ability to predict mortality. However, the HPLC assay provided information on mortality risk that the IN assay did not capture. We conclude that albuminuria, detected by either IN or HPLC, independently predicts mortality in a community-based sample. HPLC identifies some people at increased risk of mortality that IN would not detect.

Apparent loss of urinary albumin during long-term frozen storage: HPLC vs immunonephelometry

BACKGROUND

Urinary albumin detection by immunonephelometry is decreased by approximately 30% in samples that have been frozen at -20 degrees C. An HPLC method for assessment of urinary albumin that detects immunoreactive and immunochemically nonreactive albumin has been introduced as an alternative to immunonephelometry. We investigated whether this technique is affected by sample temperature, particularly freezing.

METHODS

Urine samples (n = 295) were collected from the general population (Prevention of Renal and Vascular End-Stage Disease Study). Samples were assessed by both immunonephelometry and HPLC when fresh and after storage at -20 degrees C for 4, 8, and 12 months and at -80 degrees C for 12 months.

RESULTS

With immunonephelometry, storage for 4, 8, and 12 months at -20 degrees C resulted in mean (SD) urine albumin changes of -21% (29%), -28% (29%), and -34% (31) (P <0.001 for trend). Storage at -80 degrees C resulted in a 5% (19%) change after 12 months of storage (not significant). With HPLC, storage for 4, 8, and 12 months at -20 degrees C resulted in urine albumin changes of -33% (28%), -43% (24%), and -55% (21%; P <0.001 vs immunonephelometry). Storage at -80 degrees C resulted in a -29% (29%) change (P <0.001 vs immunonephelometry).

CONCLUSION

Loss of albumin after freezing urine depends not only on freezing temperature but also on detection method. Detection of albumin by immunonephelometry appears to be significantly less influenced by freezing than detection by HPLC. Storage at -80 degrees C appears to prevent loss when using immunonephelometry, whereas HPLC still shows considerable loss even when urine is frozen at -80 degrees C. We propose that for reliable measurement of urine albumin, fresh samples should be used.