During pregnancy, abnormal proteinuria is defined as urine protein excretion greater than 300 mg/24 h. Although widely accepted, this definition is not based on clinical outcomes. Our study aimed to longitudinally examine proteinuria in healthy women prior to, and in late pregnancy and to compare inpatient and outpatient 24-hour urine collections. Nulliparous women planning to conceive were recruited and completed a 24-hour urinary collection. Those who subsequently conceived completed a second 24-hour urinary collection in late pregnancy. In the first 5 years of the study, urinary collections were completed during an inpatient admission; all collections during the latter part of the study were performed as outpatients. Urine protein was measured using the VITROS UPRO Slide kit. Wilcoxon signed rank tests were used for paired comparisons of prepregnancy and late pregnancy proteinuria and Wilcoxon rank sum tests were used to compare inpatient and outpatient collections. Among 134 women completing a prepregnancy collection, median urinary protein excretion was 188 mg/24 h (IQR 103-280). Sixty-five women subsequently conceived and completed a late pregnancy collection. In healthy women, urinary protein increased to 254 mg/24 h during pregnancy (IQR 166-396). Forty-five percent of women exceeded the defined normal threshold of proteinuria in 24 hours in the absence of disease. Inpatient collections resulted in higher levels of urinary protein than outpatient at both time points. Our data suggest that significant proteinuria is present in healthy nonpregnant women. Even in the absence of disease, proteinuria increases during pregnancy. Outpatient collections may underestimate proteinuria, especially in late pregnancy.
Keywords: 24-hour urinary protein; pregnancy; proteinuria.