How should proteinuria be detected and measured?

Chronic kidney disease and risk of major cardiovascular disease and non-vascular mortality: prospective population based cohort study

OBJECTIVE

To quantify associations of chronic kidney disease stages with major cardiovascular disease and non-vascular mortality in the general adult population.

DESIGN

Prospective population based cohort study.

SETTING

Reykjavik, Iceland.

PARTICIPANTS

16 958 people aged 33-81 years without manifest vascular disease and with available information on stage of chronic kidney disease (defined by both estimated glomerular filtration rate and urinary protein) at study entry.

MAIN OUTCOME MEASURES

Hazard ratios for time to major coronary heart disease outcomes and mortality.

RESULTS

1210 (7%) of participants had chronic kidney disease at entry. During a median follow-up of 24 years, 4010 coronary heart disease outcomes, 559 deaths from stroke, and 3875 deaths from non-vascular causes were recorded. Compared with the reference group (estimated glomerular filtration rate 75-89 ml/min/1.73 m(2) and no proteinuria), people with lower renal function within the normal range of glomerular filtration rate did not have significantly higher risk of coronary heart disease. By contrast, in 1210 (7%) participants with chronic kidney disease at entry, hazard ratios for coronary heart disease, adjusted for several conventional cardiovascular risk factors, were 1.55 (95% confidence interval 1.02 to 2.35) for stage 1, 1.72 (1.30 to 2.24) for stage 2, 1.39 (1.22 to 1.58) for stage 3a, 1.90 (1.22 to 2.96) for stage 3b, and 4.29 (1.78 to 10.32) for stage 4. Information on chronic kidney disease increased discrimination and reclassification indices for coronary heart disease when added to conventional risk factors (P<0.01). The incremental gain provided by chronic kidney disease was lower than that provided by diabetes or smoking (C index increases of 0.0015, 0.0024, and 0.0124 respectively). Hazard ratios with chronic kidney disease were 0.97 (0.82 to 1.15) for cancer mortality and 1.26 (1.07 to 1.50) for other non-vascular mortality.

CONCLUSIONS

In people without manifest vascular disease, even the earliest stages of chronic kidney disease are associated with excess risk of subsequent coronary heart disease. Assessment of chronic kidney disease in addition to conventional risk factors modestly improves prediction of risk for coronary heart disease in this population. Further studies are needed to investigate associations between chronic kidney disease and non-vascular mortality from causes other than cancer.

Tubular proteinuria in acute kidney injury: a critical evaluation of current status and future promise

The diagnosis and prognosis of acute kidney injury (AKI) by current clinical means is inadequate. Biomarkers of kidney injury that are easily measured and unaffected by physiological variables could revolutionize the management of AKI. Our objective was to systematically review the diagnostic and prognostic utility of urine and serum biomarkers of AKI in humans. We searched MEDLINE, PubMed and EMBASE databases (January 2000–August 2009) for biomarker studies that could be classified into the following categories: (a) confirmation of the diagnosis of established AKI, (b) early prediction of AKI, and (c) prognostication of AKI. We identified 54 manuscripts published since 2000 that met our inclusion and exclusion criteria. Urinary interleukin-18 (IL-18), kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL) and N-acetyl- β-d-glucosaminidase (NAG) are potentially useful biomarkers for the diagnosis of established AKI. Urinary NGAL, IL-18, and liver-type fatty acid binding protein, and serum NGAL and cystatin C represent the most promising biomarkers for early prediction of AKI. Urinary cystatin C, α1-microglobulin, NAG and retinol-binding protein may be useful to predict severity and outcomes of AKI. In conclusion, we identified several studies of promising biomarkers for the diagnosis, prediction and prognostication of AKI. However, we note several limitations, including small sample sizes, inadequate gold standard, exclusion of patients with chronic kidney disease, incomplete statistical analyses, utilization of research-based assays and a paucity of studies examining prediction for clinical outcomes. Future studies will need to address these limitations in order for further progress to be made.

Assessing proteinuria in chronic kidney disease: protein-creatinine ratio versus albumin-creatinine ratio

BACKGROUND

Quantification of proteinuria is important in the assessment of chronic kidney disease (CKD). The aim of this study was to investigate the optimal test to identify significant proteinuria.

METHODS

We retrospectively assessed the relationship between total protein:creatinine ratio (TPCR), albumin:creatinine ratio (ACR) and 24-h urine total protein in 6842 patients with CKD focusing on performance at thresholds of 0.5 and 1 g/day of proteinuria.

RESULTS

The relationship between ACR and TPCR is non-linear. TPCR is highly correlated with 24-h urine protein (Spearman's rho = 0.91), though ACR also performs well (rho = 0.84). Using receiver-operator characteristic curve analysis, TPCR outperforms ACR at predicting 0.5 g/day [area under the curve (AUC) 0.967 vs 0.951, P < 0.001] and 1 g/day of proteinuria (AUC 0.968 vs 0.947, P = 0.004). A TPCR threshold of 100 mg/mmol had a higher sensitivity (94% vs 79%) but lower specificity (88% vs 95%) than an ACR of 70 mg/mmol to predict 1 g/day of total proteinuria. To achieve comparable sensitivity, the ACR threshold falls to 17.5 mg/mmol, with lower specificity than TPCR (69.8%). Sensitivity of TPCR rose with increasing age, and in females: to achieve 95% sensitivity in a man <49 years, requires a TPCR of 65 mg/mmol, compared to 179 mg/mmol in a woman >79 years. Non-albumin proteinuria was a lower proportion of total proteinuria in patients receiving angiotensin-converting enzyme inhibitors or angiotensin receptor blockade than in those who were not (P < 0.001).

CONCLUSIONS

TPCR is a more sensitive screening test than ACR to predict clinically relevant proteinuria. The diagnostic performance of both tests varies substantially with age and gender, and should be taken into consideration when interpreting results. Total proteinuria cannot be adequately predicted from ACR, and our results suggest that caution is appropriate before utilizing ACR in patients with non-diabetic CKD.

Overview of the Alberta Kidney Disease Network

Background

The Alberta Kidney Disease Network is a collaborative nephrology research organization based on a central repository of laboratory and administrative data from the Canadian province of Alberta.

Description

The laboratory data within the Alberta Kidney Disease Network can be used to define patient populations, such as individuals with chronic kidney disease (using serum creatinine measurements to estimate kidney function) or anemia (using hemoglobin measurements). The administrative data within the Alberta Kidney Disease Network can also be used to define cohorts with common medical conditions such as hypertension and diabetes. Linkage of data sources permits assessment of socio-demographic information, clinical variables including comorbidity, as well as ascertainment of relevant outcomes such as health service encounters and events, the occurrence of new specified clinical outcomes and mortality.

Conclusion

The unique ability to combine laboratory and administrative data for a large geographically defined population provides a rich data source not only for research purposes but for policy development and to guide the delivery of health care. This research model based on computerized laboratory data could serve as a prototype for the study of other chronic conditions.