AbstractAimDipstick results for proteinuria are affected by urine concentration, and thus urine creatinine concentration ([Cr]). This study was performed to determine whether spot urine [Cr] changes significantly during pregnancy, leading to a significantly different false‐negative rate (FNR) on dipstick test between trimester.MethodsThe [Cr] and protein concentrations ([P]) were analyzed in 631 spot urine samples with negative/equivocal dipstick from 425 pregnant women. False‐negative dipstick was defined as [P] : [Cr] ratio (P/Cr) > 0.27 mg/mg.ResultsMedian [Cr] was 117 mg/dL (range, 6.5–326 mg/dL), 72 mg/dL (range, 4.3–477 mg/dL), and 73 mg/dL (range, 8.4–396 mg/dL) in the first (n = 96), second (n = 344), and third (n = 191) trimester urine samples, respectively (P = 0.000, Kruskal–Wallis). Both [P] and P/Cr increased significantly with advancing gestation. FNR 9.4% (18/191) in the third trimester was significantly higher than that of 0.0% (0/96) in the second trimester and that of 0.5% (2/344) in the third trimester. In the 20 urine samples with false‐negative dipstick, median [Cr] was 47.0 mg/dL (range, 11.0–358 mg/dL) and the proportion of samples with dilute urine, that is, [Cr] <47 mg/dL, was significantly higher than in the remaining 611 urine samples (50%, 10/20 vs 28%, 174/611, respectively, P = 0.046).ConclusionsUrine samples in the second and third trimesters were more likely to be diluted compared with the first trimester. This was associated with high FNR in third trimester urine samples.