Spot Urine Tests for Proteinuria in Suspected Preeclampsia Show Promise

August 10, 2012

The diagnostic value of the maternal “spot urine” test in suspected preeclampsia is promising, but clinical questions abound.

The diagnostic value of the maternal “spot urine” test to estimate the protein-to-creatinine ratio for significant proteinuria in suspected preeclampsia is promising, but there is insufficient evidence to determine just how the protein-to-creatinine ratio should be used in the clinical setting, mostly because of heterogeneity in test accuracy and prevalence across studies, concludes a new review study.1

The spot protein-to-creatinine ratio and albumin-to-creatinine ratio have been studied extensively outside of pregnancy, and there is sufficient evidence of a strong association between random protein-to-creatinine ratio and 24-hour protein excretion. However, there is no consensus on the most appropriate threshold to be used in clinical practice for the identification of proteinuria, according to the study authors. Therefore, the study authors examined the diagnostic accuracy of the protein-to-creatinine ratio and albumin-to-creatinine ratio compared with 24-hour urine collection for the detection of significant proteinuria in patients with suspected preeclampsia and evaluated the test results’ predictive value for maternal and fetal adverse outcomes.

For the protein-to-creatinine ratio, no threshold gave a summary estimate above 80% for both sensitivity and specificity, but threshold values between 0.22 and 0.40 gave summary estimates above 70% for both sensitivity and specificity. Therefore, the optimum threshold seems to be between 0.30 and 0.35, which is correlated with sensitivity and specificity estimates of more than 75%. This test holds value in patients with suspected preeclampsia, but how this test should be used in clinical practice remains unclear.

Because of too much variability in thresholds and study characteristics, the researchers were unable to perform a meta-analysis on results of studies of albumin-to-creatinine ratio, although one study showed this test to have diagnostic promise.2 The results were a sensitivity of 0.94, a specificity of 0.94, a positive likelihood ratio of 14.65, and a negative likelihood ratio of 0.07.2
 These study findings were consistent with those of the only other published review in this area.3 However, the results of this new study, according to its authors, are more robust.

Pertinent Points:
- The optimum threshold for the spot urine protein-to-creatinine ratio to detect proteinuria of more than 0.3 g/d is between 0.30 and 0.35.
- There is insufficient evidence for how protein-to-creatinine ratio should be used in clinical practice.
- Before widespread implementation of protein-to-creatinine ratio, clinicians must understand the prevalence of significant proteinuria in the population and setting under investigation and the most appropriate cut-off value to limit maternal and fetal morbidity and mortality to allow for accurate interpretation of the test results, advise the study authors.

References:

1. Morris RK, Riley RD, Doug M, et al. Diagnostic accuracy of spot urinary protein and albumin to creatinine ratios for detection of significant proteinuria or adverse pregnancy outcome in patients with suspected pre-eclampsia: systematic review and meta-analysis. BMJ. 2012;345:e4342.
2. Waugh J, Bell SC, Kilby MD, et al. Urine protein estimation in hypertensive pregnancy: which thresholds and laboratory assay best predict clinical outcome? Hypertens Pregnancy. 2005;24:291-302.
3. Cote AM, Brown MA, Lam EM, et al. Diagnostic accuracy of urinary spot protein:creatinine ratio for proteinuria in hypertensive pregnant women: systematic review. BMJ. 2008;336:1003-1006.