Urinary protein-to-creatinine ratio in pregnant women after dipstick testing: prospective observational study

Abstract

BackgroundThe dipstick test is widely used as a primary screening test for detection of significant proteinuria in pregnancy (SPIP). However, it often shows a false positive test result. This study was performed to determine which pregnant women should be recommended to undergo determination of urinary protein-to-creatinine ratio (mg/mg, P/Cr test) after dipstick test for confirmation of SPIP.MethodsThis was a multicenter, prospective, and observational study of 2212 urine specimens from 1033 pregnant women who underwent simultaneous dipstick and P/Cr tests in the same spot urine samples at least once. SPIP was defined as P/Cr > 0.27. Preeclampsia was diagnosed in women with both hypertension and SPIP.ResultsPreeclampsia, hypertension alone, and SPIP alone developed in 202 (20 %), 73 (7.1 %), and 120 (12 %) women, respectively. Creatinine concentration [Cr] varied greatly, ranging from 8.1 to 831 mg/dL in the 2212 urine samples. Rate of positive dipstick test results increased with increasing [Cr], while SPIP prevalence rate was lower in urine samples with higher [Cr], yielding higher false positive rates in samples with higher [Cr]. Postpartum urine samples had significantly lower [Cr] compared to those obtained antepartum (60 [8.7–297] vs. 100 [10–401] mg/dL, respectively). At the first P/Cr test among women with similar dipstick test results, the risk of having SPIP was consistently and significantly higher for hypertensive women than for normotensive women at any dipstick test result: 18 % (14/77) vs. 3.2 % (8/251), 47 % (26/55) vs. 8.7 % (37/425), 91 % (82/90) vs. 59 % (44/75) for negative/equivocal, 1+, and ≥ 2+ test results, respectively. The risk of SPIP was 16 % (9/55) for normotensive women when two successive antenatal urine samples showed a dipstick test result of 1 + .ConclusionsFor prediction of SPIP, the dipstick test was more likely to show a false positive result in concentrated urine samples with higher [Cr]. Hypertensive women with ≥ 1+ as well as normotensive women with ≥ 2+ on dipstick test should be advised to undergo the P/Cr test.

Isolated gestational proteinuria preceding the diagnosis of preeclampsia – an observational study

“…Convenient P/Cr testing using random urine samples has only recently been introduced in obstetric practice as an alternative to 24‐h urine collection . The rate of false‐positive test results on dipstick tests is high because of the inherent nature of this primary screening tool for SPIP, especially in concentrated urine samples . Although 24‐h urine collection has been mandatory for diagnosis of SPIP, it is frequently incomplete and inconvenient for pregnant women ; therefore, physicians may be reluctant to ask women for repeated 24‐h urine collection.…”

Effect of urine creatinine level during pregnancy on dipstick test

“…Therefore, it is necessary to know under what conditions the dipstick test is likely be a false negative. Spot urine P/Cr ratio is significantly correlated with daily proteinuria on 24‐h urine collection, and is relatively constant in a given day, but spot urine [Cr] varies greatly by more than 10‐fold: spot urine [Cr] ranged from 8.1 to 862 mg/dL (median, 105 mg/dL) in our previous study . Given that the dipstick test reflects [P], but not [Cr], it was expected that the FNR on dipstick test would be higher in urine samples with lower [Cr], such as those obtained during second and third trimesters than in those with higher [Cr], such as in the first trimester.…”

“…Therefore, it is necessary to know under what conditions the dipstick test is likely be a false negative. Spot urine P/Cr ratio is significantly correlated with daily proteinuria on 24-h urine collection, and is relatively constant in a given day, 7 but spot urine [Cr] varies greatly by more than 10-fold: 6 <47 mg/dL were considered as dilute urine samples for pregnant women. Dilute urine, however, was seen in 50% (10/20) of women with false-negative dipstick in this study.…”

“…Obstetric service providers often encounter false positives on dipstick test . A false‐positive result, which is likely to occur in more concentrated spot urine samples with higher [Cr], is less clinically problematic than a false‐negative result because the latter can delay PE diagnosis. This observational study was performed to determine whether [Cr] changes significantly during pregnancy.…”

“…Daily urinary creatinine excretion is relatively constant over time in a given person, 4 ranging from 11.0 mg/kg/day to 25.0 mg/kg/day, 5 but [Cr] in spot urine samples varies from <10 mg/dL to >800 mg/dL. 6 In a given person with a certain protein loss per day, given that [P] in spot urine sample increases with increasing [Cr], 7 use of [P] : [Cr] ratio (P/Cr) in spot urine is currently recommended for evaluation of protein loss per day as an alternative to 24-h urine collection outside pregnancy, 8,9 as well as in obstetric practice, [10][11][12] and a threshold of 30 mg/mmol (0.265 mg/mgCr) is recommended. [10][11][12][13] Thus, dipstick test is dependent on [P], and the false-negative rate (FNR) on dipstick test may be higher in more dilute spot urine samples with lower [Cr].…”