Diabetes with kidney injury may change the abundance and cargo of urinary extracellular vesicles

BACKGROUND

Urinary extracellular vesicles (uEVs) are derived from epithelia facing the renal tubule lumen in the kidney and urogenital tract; they may carry protein biomarkers of renal dysfunction and structural injury. However, there are scarce studies focusing on uEVs in diabetes with kidney injury.

MATERIALS AND METHODS

A community-based epidemiological survey was performed, and the participants were randomly selected for our study. uEVs were enriched by dehydrated dialysis method, quantified by Coomassie Bradford protein assay, and adjusted by urinary creatinine (UCr). Then, they identified by transmission electron microscopy (TEM), nanoparticle track analysis (NTA), and western blot of tumor susceptibility gene 101.

RESULTS

Decent uEVs with a homogeneous distribution were finally obtained, presenting a membrane-encapsulated structure like cup-shaped or roundish under TEM, having active Brownian motion, and presenting the main peak between 55 and 110 nm under NTA. The Bradford protein assay showed that the protein concentrations of uEVs were 0.02 ± 0.02, 0.04 ± 0.05, 0.05 ± 0.04, 0.07 ± 0.08, and 0.11 ± 0.15 μg/mg UCr, respectively, in normal controls and in prediabetes, diabetes with normal proteinuria, diabetes with microalbuminuria, and diabetes with macroproteinuria groups after adjusting the protein concentration with UCr by calculating the vesicles-to-creatinine ratio.

CONCLUSION

The protein concentration of uEVs in diabetes with kidney injury increased significantly than the normal controls before and after adjusting the UCr. Therefore, diabetes with kidney injury may change the abundance and cargo of uEVs, which may be involved in the physiological and pathological changes of diabetes.

Diagnostic accuracy of urine dipstick testing for albumin-to-creatinine ratio and albuminuria: A systematic review and meta-analysis

Background

The accuracy of urine dipsticks to detect increased albuminuria is uncertain. We aimed to assess the diagnostic accuracy of urine dipsticks for detecting albuminuria.

Methods

A systematic review of studies that assessed the diagnostic accuracy of urine dipstick testing for detecting albuminuria has been conducted (using as reference standard the albuminuria in a 24-hour sample or the albumin-to-creatinine ratio) in Scopus, PubMed, and Google Scholar. The risk of bias of the included studies has been assessed using the Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) tool. Whenever possible, we performed meta-analyses for sensitivity and specificity. The certainty of the evidence has also been assessed using the Grading of Recommendations Assessment, Development, and Evaluation methodology.

Results

A total of 14 studies have been included in this review, having assessed all albumin-to-creatinine ratio (ACR) as assessed standard. Each study used different dipstick types. The resulting pooled sensitivity and specificity for each cutoff point were as follows: for ACR >30 mg/g (13 studies): 0.82 (95% confidence interval: 0.76–0.87) and 0.88 (0.83–0.91); for ACR 30–300 mg/g (7 studies): 0.72 (0.68–0.77) and 0.82 (0.76–0.89); and for ACR >300 mg/g (7 studies): 0.84 (0.71–0.90) and 0.97 (0.95–0.99), respectively. An overall high risk of bias, an important heterogeneity in all pooled analysis, and a very low certainty of the evidence have been found.

Conclusions

Pooled sensitivity and specificity of urine dipsticks have been calculated for different ACR cutoff points. However, the dipstick types differed across studies, and the certainty of the evidence was very low. Thus, further well-designed studies are needed to reach more confident estimates and to assess accuracy differences across dipstick types.

Registration

PROSPERO (CRD42019124637).

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.

Storage Time and Urine Biomarker Levels in the ASSESS-AKI Study

BACKGROUND

Although stored urine samples are often used in biomarker studies focused on acute and chronic kidney disease, how storage time impacts biomarker levels is not well understood.

METHODS

866 subjects enrolled in the NIDDK-sponsored ASsessment, Serial Evaluation, and Subsequent Sequelae in Acute Kidney Injury (ASSESS-AKI) Study were included. Samples were processed under standard conditions and stored at -70°C until analyzed. Kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), interleukin-18 (IL-18), and liver fatty acid binding protein (L-FABP) were measured in urine samples collected during the index hospitalization or an outpatient visit 3 months later. Mixed effects models were used to determine the effect of storage time on biomarker levels and stratified by visit.

RESULTS

Median storage was 17.8 months (25-75% IQR 10.6-23.7) for samples from the index hospitalization and 14.6 months (IQR 7.3-20.4) for outpatient samples. In the mixed effects models, the only significant association between storage time and biomarker concentration was for KIM-1 in outpatient samples, where each month of storage was associated with a 1.7% decrease (95% CI -3% to -0.3%). There was no relationship between storage time and KIM-1 levels in samples from the index hospitalization.

CONCLUSION

There was no significant impact of storage time over a median of 18 months on urine KIM-1, NGAL, IL-18 or L-FABP in hospitalized samples; a statistically significant effect towards a decrease over time was noted for KIM-1 in outpatient samples. Additional studies are needed to determine whether longer periods of storage at -70°C systematically impact levels of these analytes.

Consecutive first-morning urine samples to measure change in the albumin-to-creatinine ratio: a pilot study of a home urine collection protocol

Background

Multiple first-morning urine samples are recommended for measuring the urine albumin-to-creatinine ratio (ACR); however, this can be challenging in community-based research.

Methods

The objectives of the study are to pilot-test a home urine collection protocol and examine how the average and variance of ACR varied with the number of urine collections and time to laboratory analysis. This is a prospective observational pilot study. This study was conducted in London, Ontario, Canada at the London Health Sciences Centre (2012–2013). The patients were adults with chronic kidney disease (mean estimated glomerular filtration rate, 36 mL/min/1.73 m²). Participants collected a first-morning 20-mL urine sample on three consecutive days. This process was repeated after 3 months. Samples were picked up by hospital courier and analyzed for ACR on the same day; additional aliquots were analyzed after a delay of 24–48 h (stored at 4 °C) and 3–9 months (stored at –80 °C). The geometric mean of the percentage change in ACR between baseline and 3 months was calculated and compared between single samples and the average of two vs. three consecutive samples.

Results

Of 31 patients enrolled, 26 (83.9 %) submitted all six urine samples. The geometric mean of ACR for three consecutive samples at baseline was 87, 83, and 80 mg/mmol, and the corresponding percentage increase from baseline to 3 months was 15 % (95 % confidence interval (CI), −9 to 46 %), 33 % (95 % CI, 10 to 59 %), and 22 % (95 % CI, −6 to 57 %). Compared with single urine collections at baseline and follow-up, averaging ACR values from two consecutive first-morning urine samples improved the sample variance and reduced the required sample size to detect a given treatment effect by approximately 30 %. No further gain in statistical efficiency was achieved with three urine samples. Results were similar when the laboratory analysis was delayed by 24–48 h, but a delay of 3–9 months resulted in systematic overestimation of the ACR. Our study’s generalizability is limited by its small sample size and reliance on a clinic-based population from a single urban center.

Conclusions

We successfully used a home urine collection protocol to obtain multiple first-morning urine samples in patients with chronic kidney disease. Statistical efficiency was improved by averaging ACR values from two consecutive first-morning urine samples at baseline and follow-up.

Effect of storage time at -20°C on markers used for assessment of renal damage in children: albumin, γ-glutamyl transpeptidase, N-acetyl-β-D-glucosaminidase and α1-microglobulin

OBJECTIVE

The objective of this study is to examine the influence of storage time at -20°C on the concentration of albumin, γ-glutamyl transpeptidase (γ-GT), N-acetyl-β-D-glucosaminidase (NAG), α(1)-microglobulin (A1M) and creatinine in a large sample of healthy children.

MATERIAL AND METHODS

The New England Children's Amalgam Trial followed 534 children, aged 6-10 at baseline, for 5 years, with annual urine collections. Urine samples were analysed for creatinine, albumin, γ-GT, NAG and A1M concentrations. Repeated measures analysis of covariance was used to model the effect of storage time on these concentrations.

RESULTS

The γ-GT concentration decreased significantly with storage time at -20°C. There was also a limited decrease in NAG. Albumin, A1M and creatinine concentrations did not appear to be affected by storage time at -20°C.

CONCLUSIONS

If it is necessary to interpret results from samples stored for a long time at -20°C, it is advisable to account for storage time in statistical models.

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.

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.

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.

Possible origins of undetectable EPO in urine samples

BACKGROUND

In order to determine the possible origins of undetectable EPO profiles in athletes' urine, we analyzed the data obtained from a large number of official anti-doping urine tests aimed at detecting recombinant erythropoietin. The following variables were considered as potential causes for lack of EPO detection: athlete's gender, competition effect, urine specific gravity as well as possible usage of proteasic adulterants to evade doping detection.

RESULTS

Statistical analyses indicated that undetectable EPO profiles were clearly related to urine properties such as low EPO concentrations or extreme specific gravities. The addition of very small quantities of protease was shown to remove all traces of EPOs in urine. This finding led to the development of a simple, specific and sensitive test that reveals proteasic activity based on albumin digestion.

CONCLUSIONS

Urine characteristics clearly affect the detectability of an EPO profile. At the same time, addition of anti-proteases prevents the adulteration of urine. These two findings have clear practical implications with regards to the timing of urine collection as well as the entire anti-doping control procedure.

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.

Electrophoretic analysis of urinary proteins in diabetic adolescents

Pathological changes in the urine sodium dodecyl sulphate gel electrophoresis (SDS PAGE) patterns often precede the occurrence of any sign of renal involvement in diabetes. However, data concerning the most frequent SDS PAGE pattern of the urine in early stages of type I diabetes mellitus are controversial. In the present study an SDS PAGE technique has been used that provides an adequate sensitivity for the detection of the abnormal pattern. Urinary proteins have been analyzed by SDS PAGE in twenty two diabetic adolescents and twenty four age matched controls. Albumin concentration, and N acetyl-beta-D-glucosaminidase (NAG) activity were also measured in the same samples. There was no significant difference in urine albumin concentration and NAG activity between diabetic children and controls. However twelve patients showed an electrophoretic pattern characteristic for glomerulopathy, two had a pattern indicating tubular dysfunction and another two patients had a mixed pattern. Among the twenty four controls only three showed abnormal electrophoretic patterns. The results support the view that early stages of diabetic nephropathy may involve both glomerular and tubular dysfunction. However the exact clinical and prognostic significance of the information provided by SDS PAGE analysis remains to be elucidated.

Excessive urinary albumin levels are associated with future cardiovascular mortality in postmenopausal women

BACKGROUND

Microalbuminuria is an early predictor of cardiovascular morbidity and mortality, in both diabetic patients and hypertensive patients. Little is known about the relation of microalbuminuria to cardiovascular disease in women of the general population.

METHODS AND RESULTS

We have studied the relation of urinary albumin levels to cardiovascular mortality in a cohort study of 12 239 postmenopausal women living in Utrecht, the Netherlands. The initial age was between 52 and 67 years. Women were followed on vital status between 1976 and 1995 (168 513 women-years). Albumin was determined in the urine of 561 cases and 557 controls. Data were analyzed by using a nested case-control design. The cardiovascular mortality rate (95% CI) for women who were in the highest quintile of urinary albumin levels was 13.2/1000 years (8.1 to 20.9) compared with 2.6/1000 years (2.3 to 3.1) in women without detectable urinary albumin. The age-adjusted rate ratio (95% CI) between these groups was 4.4 (2.6 to 7.6).

CONCLUSIONS

This is the first large cohort study that confirms a predictive role of urinary albumin for the risk of future cardiovascular mortality independent of hypertension and diabetes. Our findings support the hypothesis that microalbuminuria is a reflection of vascular damage and a marker of early arterial disease in women from the general population.

Serum and urine albumin: a progress report on their measurement and clinical significance

For about 25 years, bromcresol green and bromcresol purple have been the basis for most of the measurements of serum albumin in the US and perhaps in the world. The longevity of the methods is due to their being simple, sensitive, specific, inexpensive and relatively free from interferences. The lack of change in the serum albumin methodology is balanced by two important developments. First, the recognition of the importance of serum albumin in the maintenance of good health, and the association of decreased concentrations with increased risk of morbidity and mortality. Second, the association of albuminuria with diabetic nephropathy, which without medical intervention could lead to end-stage renal disease. The development of accurate and precise methods for urinary albumin has provided a tool to physicians to extend the length and improve the quality of life of many diabetic individuals.

The use of the Micral-Test strip to identify the presence of microalbuminuria in people with insulin dependent diabetes mellitus (IDDM) participating in the EUCLID study

In IDDM, microalbuminuria (urinary albumin excretion rate (AER) of 20-200 micrograms/min) is a predictor of persistent proteinuria and diabetic nephropathy. Early intervention may prevent or reduce the rate of progression of renal complications. The Micral-Test strip can be used to establish a semi-quantitative estimate of AER. We assessed the field performance of the Micral-Test strip in detecting microalbuminuria in the EUCLID study, an European wide, 18 centre study of 530 IDDM participants, aged 20 to 59 years. People with macroalbuminuria were excluded. On entry, all participants had albumin concentrations from two overnight urine collections measured by a central laboratory, and the corresponding Micral-Test performed on the two collections locally. a cut off of > or = mg/l albumin from the first Micral-Test, to detect a centrally measured albumin concentration > or = 20 mg/l, yielded 29 (5.8%) false negative results and 58 (11.6%) false positive results (sensitivity 70%, specificity 87%). The mean AER, from two collections, was compared with the corresponding 'pooled' Micral-Test results (mean of the two readings). Receiver Operating Characteristic (ROC) curves were used to assess if there was a suitable 'pooled' Micral-Test result for screening microalbuminuria. A 'pooled' Micral-Test result (> or = 15 mg/l) was used to detect mean AER > or = 20 micrograms/min (sensitivity 78%, specificity 77%). This 'pooled cut-off' had already been used for screening on to the study and led to an over-estimate (154 vs. 77) of the true number of microalbuminuric participants on the study. In conclusion, our findings suggest that the Micral-Test strip is not an effective screening tool for microalbuminuria, using the 'pooled' result from two measurements did not improve the sensitivity of the test.