Characterization of immunochemically nonreactive urinary albumin

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.

Presence of immunounreactive albumin in the urine of diabetic patients

Recent studies have demonstrated that conventional immunochemical assays underestimate urinary albumin concentration because of the presence of immunounreactive albumin. It has been reported that intact urinary albumin in 24-hr diabetic urine samples could be detected as total concentration (immunoreactive+immunounreactive) by an HPLC method based on size exclusion chromatography. The aim of this study was to investigate urinary albumin concentration in diabetic spot urine samples by comparing the HPLC method with several other methods. The albumin concentrations on 80 diabetic spot urine specimens were measured by turbidimetric immunoassay (TIA), high performance liquid chromatography (HPLC), and a dipstick method. In addition, they were also analyzed by reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE) and native polyacrylamide gel electrophoresis (Native PAGE). The albumin concentrations derived from diabetic spot urine samples measured by the HPLC method were higher than those of the other methods except for five of 80 samples. Furthermore, the albumin concentrations analyzed by Native PAGE were higher than SDS PAGE in 61 (76.2%) of 80 samples. This study suggests the need for evaluating diabetes not only by HPLC, but also by combining it with another method.

New screening diagnostic techniques in urinalysis

New technological evolutions have enabled new diagnostic approaches in urinalysis. Urinary flow cytometry and automated microscopic pattern recognition are two new techniques that are characterised by a much better imprecision and a higher throughput as compared to conventional microscopy of the urine sediment. Although these new techniques are well suited for the routine clinical laboratory for screening and diagnostic purposes, trained technicians are still required to verify, and if necessary, to correct the results by visual microscopy. On the other hand, automated urinary test strip analysis offers analytical, clinical, and labour cost-saving advantages. Furthermore, determination of specific urinary proteins offer interesting alternatives for diagnosis. In diabetics, the clinical significance of non-immunoreactive microalbumin needs to be established. Furthermore, the determination of specific urinary proteins alpha 1 microglobulin (as a tubular marker protein), alpha 2 macroglobulin (as a haematuria location marker) and light chains (myeloma monitoring) offer interesting diagnostic perspectives. As the information content obtained by urinalysis is complex, expert systems that make use of the various chemical and morphological parameters can offer an interesting help in the interpretation.

Update on microalbuminuria as a biomarker in renal and cardiovascular disease

PURPOSE OF REVIEW

To discuss recently published papers on the potential use of albuminuria as a predictor of cardiovascular and renal disease.

RECENT FINDINGS

Recent studies indicate that screening for microalbuminuria may not only be beneficial for detection and prevention of cardiovascular and renal disease in patients with diabetes, but also in the general population. The best method for this, however, is not yet clear. Findings indicate that it is preferable to assess albumin concentration in fresh urine samples rather than in samples that have been frozen. Furthermore, a new assay for albumin assessment has become available, which detects previously undetectable immuno-unreactive albumin above and beyond immunoreactive albumin detected by classic immunochemical assays. The pros and cons of this assay are considered.

SUMMARY

Urinary albumin is a cheap, noninvasive, and easily assessable risk marker, that does not per se require a visit to a physician or health center. As such, it is a promising candidate for screening to identify subjects at high risk of cardiovascular and renal disease, even if albuminuria is not shown to be independent of other risk markers.

Prediction of diabetic nephropathy using urine proteomic profiling 10 years prior to development of nephropathy

OBJECTIVE

We examined whether proteomic technologies identify novel urine proteins associated with subsequent development of diabetic nephropathy in subjects with type 2 diabetes before evidence of microalbuminuria.

RESEARCH DESIGN AND METHODS

In a nested case-control study of Pima Indians with type 2 diabetes, baseline (serum creatinine <1.2 mg/dl and urine albumin excretion <30 mg/g) and 10-year urine samples were examined. Case subjects (n = 31) developed diabetic nephropathy (urinary albumin-to-creatinine ratio >300 mg/g) over 10 years. Control subjects (n = 31) were matched to case subjects (1:1) according to diabetes duration, age, sex, and BMI but remained normoalbuminuric (albumin-to-creatinine ratio <30 mg/g) over the same 10 years. Surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS) was performed on baseline urine samples, and training (14 cases:14 controls) and validation (17:17) sets were tested.

RESULTS

At baseline, A1C levels differed between case and control subjects. SELDI-TOF MS detected 714 unique urine protein peaks. Of these, a 12-peak proteomic signature correctly predicted 89% of cases of diabetic nephropathy (93% sensitivity, 86% specificity) in the training set. Applying this same signature to the independent validation set yielded an accuracy rate of 74% (71% sensitivity, 76% specificity). In multivariate analyses, the 12-peak signature was independently associated with subsequent diabetic nephropathy when applied to the validation set (odds ratio [OR] 7.9 [95% CI 1.5-43.5], P = 0.017) and the entire dataset (14.5 [3.7-55.6], P = 0.001), and A1C levels were no longer significant.

CONCLUSIONS

Urine proteomic profiling identifies normoalbuminuric subjects with type 2 diabetes who subsequently develop diabetic nephropathy. Further studies are needed to characterize the specific proteins involved in this early prediction.

Diagnostic potential for urinary proteomics

Urine represents a modified ultrafiltrate of plasma, with protein concentrations typically approximately 1000-fold lower than plasma. Urine’s low protein concentration might suggest it to be a less promising diagnostic specimen than plasma. However, urine can be obtained noninvasively and tests of many urinary proteins are well-established in clinical practice. Proteomic technologies expand opportunities to analyze urinary proteins, identifying more than 1000 proteins and peptides in urine. Urine offers a sampling of most plasma proteins, with increased proportions of low-molecular-weight protein and peptide components. Urine also offers enriched sampling of proteins released along the urinary tract. Although urine presents some challenges as a diagnostic specimen, its diverse range of potential markers offers great potential for diagnosis of both systemic and kidney diseases. Examples of clinical situations where this may be of value are for more sensitive detection of kidney transplant rejection or of renal toxicity of medications.

Comparative Urine Analysis by Liquid Chromatography−Mass Spectrometry and Multivariate Statistics: Method Development, Evaluation, and Application to Proteinuria

We describe a platform for the comparative profiling of urine using reversed-phase liquid chromatography-mass spectrometry (LC-MS) and multivariate statistical data analysis. Urinary compounds were separated by gradient elution and subsequently detected by electrospray Ion-Trap MS. The lower limit of detection (5.7-21 nmol/L), within-day (2.9-19%) and between-day (4.8-19%) analytical variation of peak areas, linearity (R2: 0.918-0.999), and standard deviation for retention time (<0.52 min) of the method were assessed by means of addition of seven 3-8 amino acid peptides (0-500 nmol/L). Relating the amount of injected urine to the area under the curve (AUC) of the chromatographic trace at 214 nm better reduced the coefficient of variation (CV) of the AUC of the total ion chromatogram (CV = 10.1%) than relating it to creatinine (CV = 38.4%). LC-MS data were processed, and the common peak matrix was analyzed by principal component analysis (PCA) after supervised classification by the nearest shrunken centroid algorithm. The feasibility of the method to discriminate urine samples of differing compositions was evaluated by (i) addition of seven peptides at nanomolar concentrations to blank urine samples of different origin and (ii) a study of urine from kidney patients with and without proteinuria. (i) The added peptides were ranked as highly discriminatory peaks despite significant biological variation. (ii) Ninety-two peaks were selected best discriminating proteinuric from nonproteinuric samples, of which 6 were more intense in the majority of the proteinuric samples. Two of these 6 peaks were identified as albumin-derived peptides, which is in accordance with the early rise of albumin during glomerular proteinuria. Interestingly, other albumin-derived peptides were nondiscriminatory indicating preferential proteolysis at some cleavage sites.

Reevaluation by High-Performance Liquid Chromatography: Clinical Significance of Microalbuminuria in Individuals at High Risk of Cardiovascular Disease in the Heart Outcomes Prevention Evaluation (HOPE) Study

BACKGROUND

In the Heart Outcomes Prevention Evaluation Study, microalbumin and albumin levels measured by using radioimmunoassay (RIA) and less than the microalbuminuria threshold in baseline urine samples were associated independently with cardiovascular (CV) events. Conventional immunoassays may underestimate albuminuria by not detecting a mildly denatured unfragmented form of albumin, immunounreactive to conventional antibodies.

METHODS

Microalbuminuria was reanalyzed in baseline samples stored at -70 degrees C for 5,358 North American participants, 1,992 with diabetes mellitus, by using a high-performance liquid chromatography (HPLC) system that also detects immunochemically nonreactive urinary albumin.

RESULTS

The HPLC compared with RIA method identified microalbuminuria in 1,585 versus 719 participants, 809 versus 423 patients with diabetes, by using a conventionally accepted albumin-creatinine ratio (ACR) of 29 mg/g or greater (>or=3 mg/mmol) as a cutoff value. HPLC-detected microalbuminuria increased risk for the primary outcome (a composite of myocardial infarction, stroke, and CV death); unadjusted hazard ratio, 1.85 (95% confidence interval, 1.57 to 2.19). Receiver operating characteristic analysis did not differentiate between HPLC- and RIA-detected microalbuminuria as predictors of CV outcomes.

CONCLUSION

The prevalence of microalbuminuria is 2 to 3 times greater with HPLC than RIA using an ACR of 29 mg/g or greater (>or=3 mg/mmol). The optimal cutoff value for detecting CV risk in the entire study population by means of RIA was 9 mg/g (0.9 mg/mmol), and with HPLC, 32 mg/g (3.4 mg/mmol). Results from this study also show different ACR cutoff values for individuals with diabetes: RIA, 13 mg/g or greater (>or=1.4 mg/mmol); HPLC, 44 mg/g or greater (>or=5.2 mg/mmol) and without diabetes: RIA, 7 mg/g or greater (>or=0.7 mg/mmol); HPLC, 29 mg/g or greater (>or=3/1 mg/mmol). Results highlight the importance of method-dependent cutoff values in the prediction of CV events.

Chip Electrophoresis as a Method for Quantifying Total Microalbuminuria

Background: Microalbuminuria is an important prognostic marker in diabetic nephropathy and cardiovascular disease. Initially, most commercial assays used immunoreactivity to quantify microalbuminuria; however, size-exclusion HPLC demonstrated the existence of nonimmunoreactive forms of albumin that may not be detected by immunoassay. Recent liquid chromatography tandem mass spectrometry analyses suggested that size-exclusion HPLC gave higher results attributable to other urine proteins coeluting with albumin. We describe an assay that measures total microalbuminuria (immunoreactive and nonimmunoreactive) without any discernable interference from other common urine proteins.

Methods: We used an automated chip electrophoresis system that utilized microfluidic separation technology and fluorescent sample detection. Each albumin specimen was mixed with the manufacturer’s sample buffer in addition to a chicken albumin internal calibrator and then electrophoresed without additional reducing agents.

Results: With variable concentrations of bovine serum albumin normalized to a chicken albumin internal calibrator, the electrophoresis system was best fit with a polynomial (R2 = 0.9997; concentration range, 5–300 mg/L). The lower limit of detection was 5 mg/L. Interchip and intrachip variation studies conducted on patient urine demonstrated CVs of 3%–13%. The introduction of potentially interfering agents (i.e., molecular analytes, nonalbumin proteins) did not alter precision. Compared with immunoassay, the chip electrophoresis identified higher microalbuminuria concentrations in all urine samples. The method also clearly resolved the albumin peak from interfering proteins.

Conclusions: Unlike immunoassay, chip electrophoresis can detect both immunoreactive and nonimmunoreactive forms of albumin. This system is a simple, robust method to quantify microalbuminuria with good sensitivity, precision, and accuracy.

Detection and measurement of urinary protein

PURPOSE OF REVIEW

The measurement of urine total protein and albumin is central to the diagnosis and management of subjects with kidney disease and in assessing cardiovascular risk. Accurate assessment is vital to enable detection and management of the patient with proteinuria.

RECENT FINDINGS

The spot urine protein has been suggested as an acceptable alternative to 24-h urine collections. Recent studies suggest that this holds true for screening to exclude significant proteinuria (>1 g/day) but data are lacking for the quantification of proteinuria and in assessing response to therapy. For albuminuria, while 24-h urinary albumin excretion remains the gold standard, spot urine samples are appropriate for screening. The optimal technique for the laboratory determination of urinary albumin has been questioned with the high-performance liquid chromatography-based method demonstrating significantly more albumin in the urine. Population-based studies have found dramatic increases in the prevalence of microalbuminuria with the new high-performance liquid chromatography assay. Whether this extra immunounreactive albumin detected by high-performance liquid chromatography is clinically important remains to be established.

SUMMARY

Twenty-four-hour urine collection remains the gold standard for the accurate determination of both total urinary protein and albumin. Spot urine samples can be used for screening patients for albuminuria and proteinuria. The optimal method for measuring urinary albumin concentration remains to be established.

Prevalence and risk factor analysis of microalbuminuria in Japanese general population: the Takahata study

Microalbuminuria, an indicator of glomerular injury, is associated with increased risk of progressive renal deterioration, cardiovascular disease, and mortality. However, the prevalence of microalbuminuria in Japanese general population is less certain. Thus, we examined the prevalence of microalbuminuria and its associated risk factors in Japan. Subjects of this cross-sectional study were asymptomatic individuals over 40 years in Takahata, Japan. Urine albumin-creatinine ratio was calculated from a single-spot urine specimen collected in the morning. Creatinine clearance (CCr) was obtained by Cockcroft-Gault equation. Multivariate logistic regression analysis was used to determine which risk factors (i.e., age, hypertension, diabetes, obesity, and salt intake) might predict the presence of microalbuminuria. A total of 2321 subjects (mean age, 64 years; men, 1034; women, 1287) were entered into the final analysis. Among them, the prevalence of microalbuminuria, macroalbuminuria, and proteinuria by dipstick test (> or = 1+) were 317 (13.7%), 39 (1.7%), and 103 (4.4%), respectively. Age, hypertension, and diabetes were independently associated with microalbuminuria in men. In addition to the classical risk factors detected in men, estimated 24-h urinary sodium excretion and uric acid were also independently associated with microalbuminuria in women. Among the 668 subjects with renal insufficiency (CCr <60 ml/min/1.73 m(2)), the prevalence of microalbuminuria and macroalbuminuria were 119 (17.8%) and 18 (2.7%), respectively. In conclusion, microalbuminuria is prevalent across all age groups and is associated with lifestyle-related risk factors in Japanese general population. However, there are a substantial number of subjects with renal insufficiency accompanying no microalbuminuria.

Inefficacité des bandelettes urinaires dans la recherche de néphropathie incipiens chez les diabétiques

INTRODUCTION

Microalbuminuria is an early indication of diabetic nephropathy in patients with Type 1 diabetes and a marker of cardiovascular in patients with type 2 diabetes. It must therefore be assessed annually in these patients. We sought to determine whether semiquantitative determination of proteinuria with urinary dipsticks was useful for this purpose.

METHOD

This analysis of consecutive urinary samples among diabetic patients excluded those with dipstick results positive either for leukocyturia or nitrituria, to avoid false positives due to urinary infection. We assessed the reliability of the dipsticks in comparison with conventional microalbuminuria and proteinuria assays.

RESULTS

The study included 230 patients. Positive dipstick results had good positive (95.7%) and negative (93.9%) predictive values. Low levels of microalbuminuria, however - those that lead to early adjustment of treatment, were much more difficult to identify: the negative predictive value was only 73.7% and proteinuria was no longer correlated with microalbuminuria.

DISCUSSION

Urinary dipsticks cannot replace conventional assays for microalbuminuria or proteinuria.

Population prevalence of albuminuria in the Australian Diabetes, Obesity, and Lifestyle (AusDiab) study: immunonephelometry compared with high-performance liquid chromatography

BACKGROUND

Microalbuminuria is an independent risk factor for cardiovascular morbidity and mortality in the general population. Standard immunochemical urinary albumin assays detect immunoreactive albumin, whereas high-performance liquid chromatography (HPLC) detects both immunoreactive and immunounreactive albumin.

METHODS

Using data from the Australian Diabetes, Obesity, and Lifestyle cohort study of randomly selected community-based Australian adults, spot urine samples were tested for albuminuria (spot urine albumin-creatinine ratio [ACR]: normal, < 30 mg/g; microalbuminuria, 30 to 300 mg/g; and macroalbuminuria, > 300 mg/g) by using both immunonephelometry (IN) and HPLC (n = 10,010).

RESULTS

Bland-Altman analysis showed significant bias, with a greater ACR by means of HPLC, particularly at lower levels of ACR. Mean ACR was 15.8 mg/g (95% confidence interval [CI], 12.3 to 19.2) by means of IN compared with 30.0 mg/g (95% CI, 27.0 to 35.0) by means of HPLC. The prevalence of microalbuminuria was 4 times greater by means of HPLC compared with IN (20% versus 5.5%). In all demographic and comorbid subgroups associated with microalbuminuria, the prevalence of microalbuminuria increased by 2 to 4 times. A total of 1,743 subjects (17.4%) classified as normoalbuminuric by means of IN were reclassified as microalbuminuric by means of HPLC. Using multivariate logistic regression, women, patients with untreated and treated hypertension, and those with impaired glucose tolerance or diabetes were associated significantly with a change in category from normoalbuminuric to microalbuminuria by means of HPLC.

CONCLUSION

HPLC measures significantly more urinary albumin within the normoalbuminuria and microalbuminuria range, resulting in a significant increase in prevalence of microalbuminuria. Longitudinal studies are needed to determine whether the extra individuals identified by means of HPLC are at increased risk for developing hard clinical outcomes (renal and cardiovascular).

Analytical validation of an HPLC assay for urinary albumin

BACKGROUND

Microalbuminuria is the earliest clinical finding for renal disease. Diabetic individuals often produce modified forms of albumin, perhaps due to impaired lysosomal processing, that are undetectable by common immunoassays but accurately measured by HPLC.

METHODS

We evaluated the performance of a commercially available, FDA-approved HPLC assay (AusAm Biotechnologies, NY) and compare results to our immunoturbidimetric assay (ITA, Beckman-Coulter, CA) using random urine specimens from 32 nondiabetic and 60 type 1 and 2 diabetic subjects.

RESULTS

The HPLC assay was linear to 963 mg/l with a limit of detection of 6.1 mg/l. Within-run and between-run precision was <2% and 7-10%, respectively. Unpreserved urine was stable for at least 3 days at room temperature and 10 days at 4 degrees C. In both diabetic and nondiabetic subjects urinary albumin concentrations were higher by HPLC than by ITA, and many more diabetic and nondiabetic individuals were classified as microalbuminuric by HPLC than by ITA. The HPLC assay showed acceptable performance; however, because urinary albumin concentrations are higher in apparently healthy nondiabetic as well as diabetic subjects, different cutpoints will be necessary to accurately differentiate microalbuminuria.

CONCLUSIONS

Prospective studies are necessary to determine whether the HPLC assay can effectively detect microalbuminuria earlier than current assays without a concomitant increase in the false positive rate.

The analysis and characterisation of immuno-unreactive urinary albumin in healthy volunteers

OBJECTIVES

To compare the analysis of different forms of intact albumin in urine from healthy volunteers. To determine contamination by common non-albumin proteins on HPLC analysis of urinary albumin and of purified immuno-unreactive albumin.

DESIGN AND METHODS

Overnight urine samples collected from healthy volunteers were analysed for total albumin (immunoreactive plus immuno-unreactive) by HPLC and densitometry following native PAGE separation and for immunoreactive albumin by RIA. The contamination by non-albumin proteins of the HPLC analysis of urinary albumin and of immuno-unreactive albumin preparations was determined by ELISA. Immuno-unreactive albumin was tested for Co2+-binding capacity.

RESULTS AND CONCLUSIONS

HPLC analysis of healthy urine generates higher ACR values than immunological methods due to the presence of immuno-unreactive albumin. Immuno-unreactive albumin cannot be accounted for by the non-albumin urinary proteins tested. Isolated immuno-unreactive albumin is not recognised by antibodies to common urinary proteins or by an array of anti-albumin antibodies and behaves like serum albumin in terms of HPLC elution, native PAGE migration, and cobalt ion binding.

Coelution of other proteins with albumin during size-exclusion HPLC: Implications for analysis of urinary albumin

BACKGROUND

Size-exclusion HPLC has been used as an alternative to immunoassays for quantifying urinary albumin (microalbumin). Systematically higher values for the HPLC method have been proposed to result from nonimmunoreactive albumin.

METHODS

We evaluated separation of purified proteins and urinary components by size-exclusion HPLC using a Zorbax Bio Series GF-250 column eluted with phosphate-buffered saline. Urinary components eluting in the "albumin" peak were analyzed by mass spectrometry and reversed-phase HPLC.

RESULTS

Several proteins, such as transferrin, alpha1-proteinase inhibitor, alpha1-acid glycoprotein, and alpha2-HS glycoprotein, analyzed as purified components, were not resolved from albumin by size-exclusion HPLC. Peaks for other proteins, such as IgG and urinary components identified as dimers of alpha1-microglobulin and immunoglobulin light chains, overlapped with the albumin peak. Profiles of urine specimens showed variable amounts of components overlapping with albumin. Furthermore, the albumin peak obtained by size-exclusion HPLC was found by mass spectrometry and reversed-phase HPLC to contain multiple components in addition to albumin.

CONCLUSIONS

Size-exclusion HPLC does not resolve albumin from several other proteins in urine. The albumin peak resolved by this technique, although predominantly composed of albumin, contains several coeluting globulins that would contribute to overestimation of albumin concentration by size-exclusion HPLC.

Detection of urinary albumin

Microalbuminuria is an important clinical marker in patients with diabetes and cardiovascular disease. The concentration of albumin in urine has traditionally been measured by semiquantitative dipsticks or by various quantitative immunochemical methods such as immunonephelometry, immunoturbidimetry, and radioimmunoassay. However, until recently, urinary albumin not reabsorbed by proximal tubular cells was assumed to be excreted intact. Studies have now revealed that the nature of urinary albumin is complex and is excreted as a mixture of intact albumin, albumin-derived peptides that are not detected by routine dipstick and antibody-based tests, and a species of intact albumin (immunounreactive albumin), also not detected by dipstick and antibody-based tests. A new test, Accumin, based on high-performance liquid chromatography analysis, is able to detect all the immunoreactive intact albumin and immunounreactive intact albumin (total intact albumin) in urine. The advantage in the use of Accumin over a conventional dipstick test or antibody-based laboratory method for detecting microalbuminuria is that false negatives are reduced and a relatively earlier diagnosis of incipient kidney disease can be achieved. The introduction of Accumin has, therefore, highlighted the need for a global standard in the detection and measurement of microalbuminuria. By detecting all of the immunoreactive and immunounreactive intact albumin in urine, Accumin has virtually invalidated the use of dye and immunologically-based dipstick tests and immunologically-based laboratory methods in screening for microalbuminuria in diabetic patients and in identifying microalbuminuria as a risk factor for cardiovascular disease.