Biological Variation of Plasma and Urinary Markers of Acute Kidney Injury in Patients with Chronic Kidney Disease

BACKGROUND

Identification of acute kidney injury (AKI) is predominantly based on changes in plasma creatinine concentration, an insensitive marker. Alternative biomarkers have been proposed. The reference change value (RCV), the point at which biomarker change can be inferred to have occurred with statistical certainty, provides an objective assessment of change in serial tests results in an individual.

METHODS

In 80 patients with chronic kidney disease, weekly measurements of blood and urinary biomarker concentrations were undertaken over 6 weeks. Variability was determined and compared before and after adjustment for urinary creatinine and across subgroups stratified by level of kidney function, proteinuria, and presence or absence of diabetes.

RESULTS

RCVs were determined for whole blood, plasma, and urinary neutrophil gelatinase-associated lipocalin (111%, 59%, and 693%, respectively), plasma cystatin C (14%), creatinine (17%), and urinary kidney injury molecule 1 (497%), tissue inhibitor of metalloproteinases 2 (454%), N-acetyl-β-d-glucosaminidase (361%), interleukin-18 (819%), albumin (430%), and α1-microglobulin (216%). Blood biomarkers exhibited lower variability than urinary biomarkers. Generally, adjusting urinary biomarker concentrations for creatinine reduced (P < 0.05) within-subject biological variability (CVI). For some markers, variation differed (P < 0.05) between subgroups.

CONCLUSIONS

These data can form a basis for application of these tests in clinical practice and research studies and are applicable across different levels of kidney function and proteinuria and in the presence or absence of diabetes. Most of the studied biomarkers have relatively high CVI (noise) but also have reported large concentration changes in response to renal insult (signal); thus progressive change should be detectable (high signal-to-noise ratio) when baseline data are available.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Qualified kidney biomarkers and their potential significance in drug safety evaluation and prediction

The kidney is one of the major organs drug toxicity may target. Some renal safety biomarkers have been proposed to measure kidney injury and function accordingly. Despite the widespread use for diagnosis and monitoring of renal injury and function for decades, serum creatinine and blood urea nitrogen are nonspecific biomarkers with insensitive and delayed response in the clinical setting. There is an urgent need to identify and qualify novel kidney safety biomarkers that would be used to detect and predict drug-induced nephrotoxicity in preclinical toxicological studies, clinical trials and patient care in sequence. To do that, eight novel renal safety biomarkers have been well characterized and qualified for preclinical drug safety screening, and their clinical bridging validation is underway as well. Of them, some are used to detect or predict proximal tubular injury, and others are used to diagnose and monitor glomerular damage. Thus, measurement of a panel of kidney safety biomarkers in parallel would help maximally capture all potential safety signals for a more informative decision to be made in drug research and development as well as for optimal selection of the drug and its dose in clinical practice.

The association between urinary kidney injury molecule 1 and urinary cadmium in elderly during long-term, low-dose cadmium exposure: a pilot study

BACKGROUND

Urinary kidney injury molecule 1 is a recently discovered early biomarker for renal damage that has been proven to be correlated to urinary cadmium in rats. However, so far the association between urinary cadmium and kidney injury molecule 1 in humans after long-term, low-dose cadmium exposure has not been studied.

METHODS

We collected urine and blood samples from 153 non-smoking men and women aged 60+, living in an area with moderate cadmium pollution from a non-ferrous metal plant for a significant period. Urinary cadmium and urinary kidney injury molecule 1 as well as other renal biomarkers (alpha1-microglobulin, beta2-microglobulin, blood urea nitrogen, urinary proteins and microalbumin) were assessed.

RESULTS

Both before (r = 0.20; p = 0.01) and after (partial r = 0.32; p < 0.0001) adjustment for creatinine, age, sex, past smoking, socio-economic status and body mass index, urinary kidney injury molecule 1 correlated with urinary cadmium concentrations. No significant association was found between the other studied renal biomarkers and urinary cadmium.

CONCLUSIONS

We showed that urinary kidney injury molecule 1 levels are positively correlated with urinary cadmium concentration in an elderly population after long-term, low-dose exposure to cadmium, while other classical markers do not show an association. Therefore, urinary kidney injury molecule 1 might be considered as a biomarker for early-stage metal-induced kidney injury by cadmium.

Nephrotoxicity of uranium in drinking water from private drilled wells

OBJECTIVES

To investigate the association between uranium in drinking water from drilled wells and aspects of kidney function measured by sensitive urine tests.

METHODS

Three hundred and one of 398 eligible subjects (75.6%) aged 18-74 years with daily drinking water supplies from private drilled wells located in uranium-rich bedrock (exposed group) volunteered to participate along with 153 of 271 local controls (56.4%) who used municipal water. Participants responded to a questionnaire on their water consumption and general health, and provided a morning urine sample and drinking water for analysis.

RESULTS

The uranium content of well water samples (n=153) varied considerably (range <0.20-470 microg/l, median 6.7 microg/l, 5% >100 microg/l), while uranium levels in all samples of municipal water (n=14) were below the limit of quantification (0.2 microg/l). Urinary levels of uranium were more than eight times higher in exposed subjects than in controls (geometric means 38 and 4.3 ng/l, respectively; p<0.001), but their mean urine lead levels were not significantly different. There was a strong curvilinear correlation between uranium in drinking water and in urine (r2=0.66). Levels of albumin, beta(2)-microglobulin, protein HC as well as kappa and lambda immunoglobulin chains in urine from exposed and controls were similar. The N-acetyl-beta-d-glucosaminidase (NAG) activity was significantly lower in the exposed group vs. controls, possibly secondary to differential storage duration of samples from the two groups. Even in regression models adjusting for gender, age and smoking no association of uranium in water and the kidney function parameters was observed. Using uranium in urine in the entire study group as a marker of exposure, however, a tendency of exposure-related increases of beta(2)-microglobulin, protein HC and kappa chains were noted. This tendency was enhanced after exclusion of subjects with diabetes mellitus from the analysis.

CONCLUSIONS

Uranium levels in urine were strongly correlated to levels in drinking water from drilled wells. There were no clear signs of nephrotoxicity from uranium in drinking water at levels recorded in this study, but some indications of an effect were observed using uranium in urine as a measure of overall uranium exposure. The clinical relevance of these findings remains unclear.

Alpha 1-microglobulin: clinical laboratory aspects and applications

BACKGROUND

Urinary microproteins are becoming increasingly important in clinical diagnostics. They can contribute in the non-invasive early detection of renal abnormalities and the differentiation of various nephrological and urological pathologies. Alpha 1-microglobulin (A1M) is an immunomodulatory protein with a broad spectrum of possible clinical applications and seems a promising marker for evaluation of tubular function.

METHOD

We performed a systematic review of the peer-reviewed literature (until end of November 2003) on A1M with emphasis on clinical diagnostic utility and laboratory aspects.

CONCLUSIONS

A1M is a 27-kDa glycoprotein, present in various body fluids, with unknown exact biological function. The protein acts as a mediator of bacterial adhesion to polymer surfaces and is involved in inhibiting renal lithogenesis. Because A1M is not an acute phase protein, is stable in a broad range of physiological conditions and sensitive immunoassays have been developed, its measurement can be used for clinical purposes. Unfortunately, international standardisation is still lacking. Altered plasma/serum levels are usually due to impaired liver or kidney functions but are also observed in clinical conditions such as HIV and mood disorders. Urinary A1M provides a non-invasive, inexpensive diagnostic alternative for the diagnosis and monitoring of urinary tract disorders (early detection of tubular disorders such as heavy metal intoxications, diabetic nephropathy, urinary outflow disorders and pyelonephritis).

Renal tubular proteinuria in patients with irritable bowel syndrome

OBJECTIVE

Irritable bowel syndrome (IBS) is a common condition that is poorly understood. We have previously demonstrated tubular protinuria in patients with inflammatory bowel disease. This study examined whether tubular proteinuria was a feature of IBS.

METHODS

Eighty control subjects (male:female, 28:52; age range 20-65 years) and 21 patients with IBS (male:female, 9:12; age range 16-64 years) (not significant) were recruited. Patients with known renal disease, hypertension, diabetes or microbiological evidence of urinary infection were excluded. The IBS patients all fulfilled the ROME II criteria. None had preceding gastroenteritis. Urinary alpha1-microglobulin (alpha1-M) was measured in a second-voided morning urine sample and corrected for urinary concentration by measurement of creatine. Blood samples were analysed for haematochemical indices including C-reactive protein. Statistical analysis was by unpaired t test.

RESULTS

None of the IBS patients were reclassified with inflammatory bowel disease over a 5-year follow up period. All had normal haematochemical parameters. Mean +/- standard deviation urinary alpha1-M concentrations were significantly higher in IBS patients than controls (IBS patients, 1.17 +/- 0.65 mg/mmol; controls, 0.75 +/- 0.36 mg/mmol; P < 0.01) and exceeded 1.5 mg/mmol (the upper reference limit) in seven patients. There was no difference in urinary alpha1-M concentrations in the diarrhoea-predominant and constipation-predominant groups (mean +/- standard deviation, 1.342 +/- 0.65 versus 0.76 +/- 0.48 mg/mmol; P = 0.062).

CONCLUSIONS

Urinary alpha1-M concentration is commonly increased in IBS, suggesting the presence of renal proximal tubular injury.

α1-Microglobulin as a promising marker of cadmium-induced tubular dysfunction, possibly better than β2-microglobulin

The purpose of the present study was to evaluate the validity of alpha1-microglobulin (alpha1-MG) in comparison with popularly used beta2-microglobulin (beta2-MG). A database on 8975 cases of never-smoking adult women was revisited; the data were based on spot urine samples from the women in 10 prefectures all over Japan. The validity of alpha1-MG was examined following essentially the same protocol as beta2-MG was examined in a previous study. Comparisons were made for alpha1-MG as observed (e.g. alpha1-MG(ob)), as corrected for creatinine (CR or cr) (e.g. alpha1-MGcr) and as corrected for a specific gravity (SG or sg) of 1.016 (e.g. alpha1-MGsg). A cut-off value of 5.0 mg alpha1-MG/g cr or l was deduced from 400 microg beta2-MG/g cr taking advantage of the regression equation between alpha1-MG and beta2-MG. The prevalence of alph1-microglobulinuria as corrected for a specific gravity of 1.016 (or alpha1-MGsg-uria in short) was essentially unchanged irrespective of SG, except for in very dense or very thin urine samples. alpha1-MGcr-uria prevalence decreased at higher CR. Comparison of the present observation with previous findings on beta2-MG-uria prevalence showed that the variation in prevalence of MG-uria as a function of urine density was smaller for alpha1-MGsg whereas it was substantially larger for beta2-MGcr, and thus it appeared prudent to consider alpha1-MGsg rather than beta2-MGcr as a marker of tubular dysfunction.