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

Review on adherence of the literature to official recommendations on albuminuria harmonization and standardization

Albuminuria standardization is a key issue to produce reliable and equivalent results between laboratories. We investigated whether official recommendations on albuminuria harmonization are followed in the literature. The PubMed database was searched from June 1 to September 26, 2021. The search terms included urine albumin, urine albumin-to-creatinine ratio (uACR), and albuminuria. A total of 159 articles were considered eligible; 50.9 % reported the type of urine collection. Specifically, 58.1 % collected a random spot urine specimen, 21 % collected a first morning void, and 6.2 % collected a 24-h specimen. Overall, 15 % of articles reported data on sample shipping, storage, and centrifugation and 13.3 % mentioned the preanalytical phase without any data on albuminuria. The method for albuminuria was properly described in 31.4 % of articles; of these, 54.9 % used immunological methods, and 8.9 % contained errors or missing data. Most articles (76.7 %) expressed test results as albuminuria-to-creatininuria ratio. Different decision levels were utilized in 130 articles; of these, 36 % used a decision level of ≤30 mg/g creatininuria and 23.7 % used three decision levels (≤30, 30-300, and ≥300 mg/g). The failure to follow guidelines on albuminuria harmonization was mainly found in the preanalytical phase. The poor awareness of the importance of preanalytical steps on test result may be a possible explanation.

Immune-unreactive urinary albumin as a predictor of cardiovascular events: the Hortega Study

BACKGROUND

We aimed to determine if immune-unreactive albumin excretion (IURAE) is associated with cardiovascular (CV) events in a representative sample of a general population from Spain.

METHODS

We included 1297 subjects (mean age ± standard error 48.0 ± 0.2 years, 48% females), who participated in the Hortega Follow-Up Study. The primary endpoint was incidence of fatal and non-fatal CV events. Urinary albumin excretion (UAE) was measured in spot voided urine, frozen at -80°C, by immunonephelometry [immune-reactive albumin excretion (IRAE)] and by high-performance liquid chromatography (HPLC) [total albumin excretion (AE)]. IURAE was calculated as the difference between HPLC measurements and IRAE. We estimated fully adjusted hazard ratios (HRs) of CV incidence by Cox regression for IRAE, IURAE and total AE.

RESULTS

After an average at-risk follow-up of 13 years, we observed 172 CV events. urinary albumin to creatinine ratio (UACR) of ≥30 mg/g assessed by IRAE, IURAE or total AE concentrations was observed in 74, 273 and 417 participants, respectively. Among discordant pairs, there were 49 events in those classified as micro- and macroalbuminuric by IURAE, but normoalbuminuric by IRAE. Only the IRAE was a significant independent factor for the incidence of CV events [HR (95% confidence interval) 1.15 (1.04-1.27)]. The association of UAE with CV events was mainly driven by heart failure (HF) [HR 1.33 (1.15-1.55) for IRAE; HR 1.38 (1.06-1.79) for IURAE; HR 1.62 (1.22-2.13) for total AE]. Those subjects who were micro- and macroalbuminuric by both IRAE and IURAE had a significant increase in risk for any CV event, and especially for HF.

CONCLUSIONS

IRAE, IURAE and AE were associated with an increased risk for CV events, but IRAE offered better prognostic assessment.

Stability of urinary albumin and creatinine after 12 months storage at -20 °C and -80 °C

Background

Increasing albumin to creatinine ratio (ACR) within the normal range is a risk factor for cardiovascular disease in the general population. Clinical and epidemiological studies often store urine samples for long durations prior to ACR assessment. The stability of ACR at the lowest urinary albumin concentrations during prolonged storage has not been previously studied because routine clinical assays can’t quantify very low concentrations of albumin.

Aim

To determine the stability of urinary albumin and creatinine over 12 months in samples stored at −20 °C and −80 °C using an assay which enables assessment of previously undetectable levels of albumin and to investigate if additives can be used to prevent urinary albumin degradation.

Method

ACR was measured in 30 urine samples from healthy subjects on the day of collection. Each sample was divided into 5 portions, each receiving a different treatment; alkalisation, protease inhibiter, boric acid, low protein binding tubes and no treatment (control). Samples were stored at −20 °C and −80 °C and ACR was analysed again after 12 months.

Results

Mean (95% CI) percent change in ACR was −34.3% (−47.2 to −21.4; p < 0.0001) and −1.8% (−9.4 to 5.8; p = 0.91) in samples stored at −20 °C and −80 °C respectively. Treating samples did not prevent the reduction in albumin at −20 °C (p < 0.001).

Conclusion

The loss in urinary albumin concentration which occurs during storage at −20 °C for 12 months is not prevented by pre-treating samples prior to storage. For accurate determination of albumin concentration or ACR, samples should be stored at −80 °C on day of collection.

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Urine samples should be stored at −80 °C on day of collection to prevent loss of albumin.

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ACR is reduced by over 30% in samples stored at −20 °C for 12 months.

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Treating samples prior to storage at −20 °C does not prevent loss in urinary albumin.

Effect of common storage temperatures and container types on urine protein : creatinine ratios in urine samples of proteinuric dogs

Background

Preanalytic protein adsorption to polymer and glass container surfaces may decrease urine protein concentration measurements and urine protein: creatinine ratios (UPC).

Hypothesis/Objectives

Urine stored in PC or glass containers will have lower UPC than urine stored in HP containers. The specific objective was to determine whether clinically relevant differences in UPC would be detected after storage in glass, PC, or HP containers using common storage times and temperatures.

Animals

Twelve client‐owned dogs with proteinuria.

Methods

Prospective, nonmasked study, divided into 2 phases. The first phase was a pilot study involving multiple (n = 5) measurements at each storage condition using 24‐hours urine samples from 2 dogs with persistent renal proteinuria of different magnitude. The second phase used urine samples from 10 dogs with proteinuria of variable magnitude. Sample aliquots were stored in HP, PC, and glass containers at 24°C for 4 hours, 4°C for 12 hours, and −20°C for 72 hours. The UPC of each was measured after storage and compared with baseline.

Results

Statistically significant but clinically irrelevant differences were found in phase 1. In phase 2, storage conditions did not affect urinary protein or creatinine concentrations or UPC.

Conclusions and Clinical Importance

Collection and storage of canine urine samples in clean HP, PC, or glass containers at 24°C for 4 hours, 4°C for 12 hours, or −20°C for 72 hours is unlikely to result in clinically relevant decreases in measured UPC values.

Screening for elevated albuminuria and subsequently hypertension identifies subjects in which treatment may be warranted to prevent renal function decline

Background

We investigated whether initial population screening for elevated albuminuria with subsequent screening for hypertension in case albuminuria is elevated may be of help to identify subjects at risk for accelerated decline in kidney function.

Methods

We included subjects who participate in the PREVEND observational, general population-based cohort study and had two or more glomerular filtration rate (eGFR) measurements available during follow-up. Elevated albuminuria was defined as an albumin concentration ≥20 mg/L in a first morning urine sample confirmed by an albumin excretion ≥30 mg/day in two 24-h urines. Hypertension was defined as systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg or use of blood pressure-lowering drugs. eGFR was estimated with the CKD-EPI creatinine-cystatin C equation.

Results

Overall, 6471 subjects were included with a median of 4 [95% confidence interval (CI) 2-5] eGFR measurements during a follow-up of 11.3 (95% CI 4.0-13.7) years. Decline in eGFR was greater in the subgroups with elevated albuminuria. This held true, not only in subjects with known hypertension (-1.84 ± 2.27 versus -1.16 ± 1.45 mL/min/1.73 m 2 per year, P < 0.05), but also in subjects with newly diagnosed hypertension (-1.59 ± 1.55 versus -1.14 ± 1.38 mL/min/1.73 m 2 per year, P < 0.05) and in subjects with normal blood pressure (-1.18 ± 1.85 versus -0.81 ± 1.02 mL/min/1.73 m 2 per year in subjects, P < 0.05). This effect was most pronounced in the population ≥55 years of age and male subjects. In addition, subjects with elevated albuminuria had higher blood pressure than subjects with normoalbuminuria, and in subjects with elevated albuminuria as yet undiagnosed hypertension was twice as prevalent as diagnosed hypertension.

Conclusions

Initial screening for elevated albuminuria followed by screening for hypertension may help to detect subjects with increased risk for a steeper decline in kidney function.

Long-term changes in albuminuria: underlying causes and future mortality risk in a 20-year prospective cohort: the Nord-Trøndelag Health (HUNT) Study

OBJECTIVE

Knowledge on how changing risk factors influence the progression of albuminuria over time is still limited. Furthermore, large population-based cohorts are needed to study the association between albuminuria change and mortality risk in nondiabetic study participants.

METHODS

We evaluated changes of albuminuria in 6282 nondiabetic individuals from the Norwegian population-based Nord-Trøndelag Health study. Using three albumin/creatinine ratios (ACR), we studied the influence of cardiovascular risk factors on ACR change from baseline to follow-up 11 years later. We evaluated the next 8-year mortality risk by using flexible parametric methods to identify nonlinear main effects and their two-way interactions.

RESULTS

Mean albuminuria increased significantly over 11 years (1.82-3.02 mg/mmol, P < 0.0001), but two-thirds of individuals had stable levels (ΔACR -1.40 to 1.40 mg/mmol). Higher age, ACR, and SBP as well as smoking and lower glomerular filtration rate at baseline were associated with increasing albuminuria. Study participants in the upper quartile of the increasing group had mean adjusted hazard ratio 1.31 (P = 0.004) for all-cause mortality compared with those with stable ACR. Those with decreasing ACR also had increased mortality, but the risk was strongly attenuated when adjusting for comorbidity. It also decreased the first 3 years before increasing. There was a strong interaction between baseline ACR and ΔACR. Increasing albuminuria had strongest effect on mortality in study participants with moderately increased baseline values.

CONCLUSION

Both increasing and decreasing albuminuria are significant independent predictors of mortality in nondiabetic individuals, but must be interpreted in light of baseline values. Cutoffs and clinical usefulness in nondiabetic study participants should be further investigated.

Effect of Processing Delay and Storage Conditions on Urine Albumin-to-Creatinine Ratio

BACKGROUND AND OBJECTIVES

Because there is substantial biologic intraindividual variation in albumin excretion, randomized trials of albuminuria-reducing therapies may need multiple urine samples to estimate daily urinary albumin excretion. Mailing spot urine samples could offer a convenient and cost-effective method to collect multiple samples, but urine albumin-to-creatinine ratio stability in samples stored at ambient temperatures for several days is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Patients with kidney disease provided fresh urine samples in two tubes (with and without boric acid preservative). Reference aliquots from each participant were analyzed immediately, whereas remaining aliquots were subject to different handling/storage conditions before analysis, including delayed processing for up to 7 days at three different storage temperatures (4°C, 18°C, and 30°C), multiple freeze-thaw cycles, and long-term frozen storage at -80°C, -40°C, and -20°C. We calculated the mean percentage change in urine albumin-to-creatinine ratio for each condition, and we considered samples stable if the 95% confidence interval was within a ±5% threshold.

RESULTS

Ninety-three patients provided samples with detectable albuminuria in the reference aliquot. Median (interquartile range) urine albumin-to-creatinine ratio was 87 (20-499) mg/g. The inclusion of preservative had minimal effect on fresh urine albumin-to-creatinine ratio measurements but reduced the changes in albumin and creatinine in samples subject to processing delay and storage conditions. The urine albumin-to-creatinine ratio was stable for 7 days in samples containing preservative at 4°C and 18°C and 2 days when stored at 30°C. It was also stable in samples with preservative after three freeze-thaw cycles and in frozen storage for 6 months at -80°C or -40°C but not at -20°C.

CONCLUSIONS

Mailed urine samples collected with preservative and received within 7 days if ambient temperature is ≤18°C, or within 2 days if the temperature is higher but does not exceed 30°C, are suitable for the measurement of urine albumin-to-creatinine ratio in randomized trials. Preserved samples frozen to -40°C or -80°C for 6 months before analysis also seem suitable.

A basic science view of acute kidney injury biomarkers

Over the last decade, significant progress has been made in the identification and validation of novel biomarkers as well as refinements in the use of serum creatinine as a marker of kidney function. These advances have taken advantage of laboratory investigations, which have identified these novel molecules that serve important biological functions in the pathogenesis of acute kidney injury (AKI). As we advance and validate these markers for clinical studies in AKI, we recognize that they serve not only to improve our understanding of AKI, but they could also serve as potential targets for the treatment of AKI. This review will underscore the biological basis of specific biomarkers that will contribute to the advancement in the treatment and outcomes of AKI.

What is the best method of proteinuria measurement in clinical trials of endothelin receptor antagonists?

AIMS

To determine whether protein-creatinine ratio (PCR) and albumin-creatinine ratio (ACR) are comparable to 24h urine protein in terms of agreement and repeatability, and therefore whether they are suitable for monitoring and comparing reduction in proteinuria in clinical trials of endothelin receptor antagonists.

MAIN METHODS

Using data from a recent study of sitaxentan in 27 patients with proteinuric chronic kidney disease, the assays were compared with reference to their agreement, repeatability, the number of measurements required to obtain accurate results and correlation with reduction in proteinuria at baseline.

KEY FINDINGS

The median coefficient of variation was lower for PCR than 24h urine protein (25 vs. 28%) but the range was higher (70 vs. 47%). When converted into the same units, mean difference between 24h urine protein and both PCR (0.03 g/day), and ACR (0.10 g/day), was small. However, scatter increased with mean level of proteinuria, such that agreement fell substantially above 1.5 g/day. According to 2-factor within-subjects ANOVA, the assay used was not a significant source of variation (PCR p=0.63, ACR p=0.38). With 3 measurements at each time point, baseline proteinuria correlated equally well with change in proteinuria, and percentage change was detected accurately by all 3 methods.

SIGNIFICANCE

PCR and ACR may well be suitable replacements for 24h urine protein in the clinical trial context due to their similar accuracy and repeatability, greater convenience and lower cost. However, a randomised control trial comparing all 3 assays in a larger and more diverse population is necessary before 24h urine protein can be replaced.

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.

Pre-existing albuminuria predicts AIDS and non-AIDS mortality in women initiating antiretroviral therapy

BACKGROUND

We previously reported an increased risk of all-cause and AIDS mortality among HIV-infected women with albuminuria (proteinuria or microalbuminuria) enrolled in the Women's Interagency HIV Study (WIHS) prior to the introduction of HAART.

METHODS

The current analysis includes 1,073 WIHS participants who subsequently initiated HAART. Urinalysis for proteinuria and semi-quantitative testing for microalbuminuria from two consecutive study visits prior to HAART initiation were categorized as follows: confirmed proteinuria (both specimens positive for protein), confirmed microalbuminuria (both specimens positive with at least one microalbuminuria), unconfirmed albuminuria (one specimen positive for proteinuria or microalbuminuria), or negative (both specimens negative). Time from HAART initiation to death was modelled using proportional hazards analysis.

RESULTS

Compared with the reference group of women with two negative specimens, the hazard ratio (HR) for all-cause mortality was significantly increased for women with confirmed microalbuminuria (HR 1.9, 95% CI 1.2-2.9). Confirmed microalbuminuria was also independently associated with AIDS death (HR 2.3, 95% CI 1.3-4.3), whereas women with confirmed proteinuria were at increased risk for non-AIDS death (HR 2.4, 95% CI 1.2-4.6).

CONCLUSIONS

In women initiating HAART, pre-existing microalbuminuria independently predicted increased AIDS mortality, whereas pre-existing proteinuria predicted increased risk of non-AIDS death. Urine testing may identify HIV-infected individuals at increased risk for mortality even after the initiation of HAART. Future studies should consider whether these widely available tests can identify individuals who would benefit from more aggressive management of HIV infection and comorbid conditions associated with mortality in this population.

Predictors of incident albuminuria in the Framingham Offspring cohort

BACKGROUND

Predictors for incident albuminuria are not well known in population-based cohorts. The purpose of this study is to identify predictors of incident albuminuria in an unselected middle-aged population.

STUDY DESIGN

Observational cohort study.

SETTING & PARTICIPANTS

Framingham Offspring Study participants who attended both the sixth (baseline; 1995-1998) and eighth (2005-2008) examination cycles.

PREDICTORS

Standard clinical predictors were used. Predictors of incident albuminuria were identified using stepwise logistic regression analysis with age and sex forced into the model.

OUTCOMES & MEASUREMENTS

Albuminuria was defined as urine albumin-creatinine ratio (UACR) ≥ 17 mg/g (men) or ≥ 25 mg/g (women). Individuals with albuminuria at baseline were excluded.

RESULTS

1,916 participants were available for analysis (mean age, 56 years; 54% women). Albuminuria developed in 10.0% of participants (n = 192) during 9.5 years. Age (OR, 2.09; P < 0.001), baseline diabetes (OR, 1.93; P = 0.01), smoking (OR, 2.09; P < 0.001), and baseline log UACR (OR per 1-SD increase in log UACR, 1.56; P < 0.001) were associated with incident albuminuria in a stepwise model. An inverse relationship with female sex (OR, 0.53; P < 0.001) and high-density lipoprotein (HDL) cholesterol level (OR, 0.80; P = 0.007) also was observed. Results were similar when participants with baseline chronic kidney disease (n = 102), defined as estimated glomerular filtration rate <60 mL/min/1.73 m(2), were excluded from the model. Age, male sex, low HDL cholesterol level, smoking, and log UACR continued to be associated with incident albuminuria when baseline diabetes (n = 107) was excluded. Age, male sex, and log UACR correlated with incident albuminuria after participants with baseline hypertension were excluded (n = 651).

LIMITATIONS

Causality may not be inferred because of the observational nature of the study. One-third of participants did not return for follow-up, potentially attenuating the observed risks of albuminuria.

CONCLUSIONS

The known cardiovascular risk factors of increasing age, male sex, diabetes, smoking, low HDL cholesterol level, and albuminuria within the reference range are correlates of incident albuminuria in the general population.

Effect of Sampling Method and Storage Conditions on Albumin, Retinol-Binding Protein, And N-Acetyl-β-D-Glucosaminidase Concentrations in Canine Urine Samples

Urinary markers for renal dysfunction are gaining interest, but effects of sampling method, storage conditions, and urinary tract inflammation or infection on these markers are unclear. Therefore, the objectives of the current study were to determine the difference in urinary albumin (uALB), urinary retinol-binding protein (uRBP), and urinary N-acetyl-β-D-glucosaminidase (uNAG) concentrations in cystocentesis and voided samples and to investigate concentration changes after storage at −20°C and at −80°C. Effects of a protease inhibitor were also assessed in samples stored at −80°C for 12 months. In a pilot experiment, influence of in vitro hematuria, pyuria, and bacteriuria on the urinary markers was evaluated. A mixed model was used to calculate mean differences and 95% confidence intervals. Urinary ALB, uNAG, and uRBP concentrations were similar in voided and cystocentesis samples. After storage for 4 months at −20°C, uALB concentration was not affected, and uRBP concentration showed a mild and clinically irrelevant decrease, whereas uNAG activity was significantly lower compared with fresh samples. After storage for 12 months at −80°C, uALB and uRBP concentrations did not differ from fresh samples, but uNAG activity was severely decreased. Protease inhibitor addition did not preserve uNAG activity. Experimental hematuria, pyuria, and bacteriuria did not seem to affect urinary markers, although further research is needed.

Microalbuminuria is associated with all-cause and AIDS mortality in women with HIV infection

OBJECTIVES

Prevalence of microalbuminuria is increased in patients with HIV. Microalbuminuria is associated with increased mortality in other populations, including diabetics, for whom microalbuminuria testing is standard of care. We investigated whether microalbuminuria is associated with mortality in HIV-infected women not receiving antiretroviral therapy.

METHODS

Urinalysis for proteinuria and semiquantitative testing for microalbuminuria were performed in specimens from 2 consecutive visits in 1547 HIV-infected women enrolled in the Women's Interagency HIV Study in 1994-1995. Time to death was modeled using proportional hazards analysis.

RESULTS

Compared with women without albuminuria, the hazard ratio (HR) for all-cause mortality was increased in women with 1 (HR: 3.4; 95% CI: 2.2 to 5.2) or 2 specimens positive for either proteinuria or microalbuminuria (HR: 3.9; 95% CI: 2.1 to 7.0). The highest risk was observed in women with both specimens positive for proteinuria (HR: 5.8; 95% CI: 3.4 to 9.8). The association between albuminuria and all-cause mortality risk remained significant after adjustment for demographics, HIV disease severity, and related comorbidities. Similar results were obtained for AIDS death.

CONCLUSIONS

We identified a graded relationship between albuminuria and the risk of all-cause and AIDS mortality.

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.

Effect of repeated freeze-thaw cycles on urinary albumin-to-creatinine ratio

BACKGROUND

Urinary albumin-to-creatinine ratio (UACR) is widely used for diagnosis of chronic kidney disease in population investigation surveys. It is recommended that urinary albumin should be measured as soon as possible after urine is collected. It is not clear whether freezing and thawing affect the value of UACR and it is very inconvenient to measure UACR quickly in a population survey. The current study tries to determine the effect of repeated freezing and thawing on UACR to explore the possibility of freezing urine samples.

METHODS

Fifty-three urine samples with abnormal urinary albumin (ranging from 22.9 mg/L to 891.9 mg/L) were selected. The albumin and creatinine were measured before freezing, then all sample were stored at -30 degrees C. After being thawed at room temperature, the albumin and creatinine concentrations were measured again. The samples were frozen and thawed for five times, and albumin and creatinine were measured after each thawing. The measurements of albumin, creatinine, and UACR after each thawing were calculated and compared with its initial values using multiple comparisons of one-way ANOVA.

RESULTS

Compared with its initial value, urine albumin, creatinine and UACR all did not show any significant differences (p > 0.05).

CONCLUSIONS

It is feasible to freeze urine samples for future measurement of UACR. Urine samples can be safely frozen and thawed at least five times.

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.

Alkalinization of urine samples preserves albumin concentrations during prolonged frozen storage in patients with diabetes mellitus

BACKGROUND

In epidemiological studies in patients with diabetes, urine samples are often stored frozen prior to assessment of urinary albumin concentration (UAC). However, prolonged frozen storage may result in a falsely low UAC. In the current study, we investigated whether adjustment of urinary pH to alkaline values prior to frozen storage can prevent this problem.

METHODS

Urine samples were collected in 90 patients from our diabetes outpatient clinic and divided into two portions. One portion was first adjusted to pH > 8.0 with 0.1 m sodium hydroxide, the other was left unprocessed. Both portions were divided into aliquots. UAC was assessed in fresh samples and after 7 days, 1, 6 and 12 months of storage at -20 and -80 degrees C.

RESULTS

Until 1 month of storage there were no significant changes in UAC. After longer storage, UAC fell significantly in pH unadjusted samples stored at -20 degrees C, with a -7.6% (27.8) and -13.6% (31.7) change after 6 and 12 months storage, respectively. No significant change in UAC occurred in pH adjusted samples stored at -20 degrees C or when samples were stored at -80 degrees C, both with and without pH adjustment. Variation in UAC assessed after 12 months of storage was larger for samples stored at -20 degrees C without adjustment of pH than for the samples stored with pH adjustment or stored at -80 degrees C.

CONCLUSIONS

Urine alkalinization to pH > 8.0 prevents the decline in UAC associated with 12 months of frozen storage at -20 degrees C and results in lower variation between samples after storage.

Combining GFR and albuminuria to classify CKD improves prediction of ESRD

Despite the high prevalence of chronic kidney disease (CKD), relatively few individuals with CKD progress to ESRD. A better understanding of the risk factors for progression could improve the classification system of CKD and strategies for screening. We analyzed data from 65,589 adults who participated in the Nord-Trøndelag Health (HUNT 2) Study (1995 to 1997) and found 124 patients who progressed to ESRD after 10.3 yr of follow-up. In multivariable survival analysis, estimated GFR (eGFR) and albuminuria were independently and strongly associated with progression to ESRD: Hazard ratios for eGFR 45 to 59, 30 to 44, and 15 to 29 ml/min per 1.73 m(2) were 6.7, 18.8, and 65.7, respectively (P < 0.001 for all), and for micro- and macroalbuminuria were 13.0 and 47.2 (P < 0.001 for both). Hypertension, diabetes, male gender, smoking, depression, obesity, cardiovascular disease, dyslipidemia, physical activity and education did not add predictive information. Time-dependent receiver operating characteristic analyses showed that considering both the urinary albumin/creatinine ratio and eGFR substantially improved diagnostic accuracy. Referral based on current stages 3 to 4 CKD (eGFR 15 to 59 ml/min per 1.73 m(2)) would include 4.7% of the general population and identify 69.4% of all individuals progressing to ESRD. Referral based on our classification system would include 1.4% of the general population without losing predictive power (i.e., it would detect 65.6% of all individuals progressing to ESRD). In conclusion, all levels of reduced eGFR should be complemented by quantification of urinary albumin to predict optimally progression to ESRD.

Diagnostic accuracy of a reagent strip for assessing urinary albumin excretion in the general population

BACKGROUND

Albuminuria is a sensitive marker of renal derangement and has been included in a number of studies investigating chronic kidney diseases (CKDs). This study is aimed to evaluate the diagnostic performances of a strip for measuring the albumin/creatinine ratio (ACR) in the general population and to compare it with those found in a diabetic population.

METHODS

Urine samples were obtained from 201 consecutive subjects enrolled in an epidemiological study and from 259 type 2 diabetic patients. Urine was tested for albumin and creatinine using the strip (Clinitek Microalbumin) and laboratory methods. A hundred samples were stored under various conditions to assess analyte stability.

RESULTS

In the general population, the strip test reached a 90% sensitivity and 91% specificity, considering the laboratory method as the 'gold standard', sparing >80% of subjects the laboratory tests at the expense of a 1% false negative rate and an 8% false positive rate. Regarding sensitivity and specificity, the ACR test performs very similarly in the general population and in the diabetics. The stability study showed that storage at -20 degrees C induced a significant decrease in the albumin concentration with both methods, such that 5% of the samples were re-classified in the lower ACR class. Storage at -80 degrees C for up to 12 months did not affect the measurement with both methods.

CONCLUSION

Clinitek Microalbumin strips can be used for screening purposes in the general population since they correctly classify a significant percentage of subjects, particularly those with a normal albuminuria. Storage at -80 degrees C does not affect strip results. Screening with the strip and confirming positive results with a wet chemistry method are an efficient strategy for detecting albuminuria in the general population.

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.

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.