OBJECTIVE: To systematically review the literature and synthesize data on the diagnostic performance of a 12-hour urine collection for proteinuria in pregnant women with suspected preeclampsia. DATA SOURCES: We performed a literature search of PubMed, Embase, Scopus, ClinicalTrials.gov, and CINAHL through February 2014 using key words related to gestational hypertension, preeclampsia, and proteinuria. METHODS OF STUDY SELECTION: Studies that contained results of both the 12-hour and 24-hour urine collection in the same patients were eligible. TABULATION, INTEGRATION, AND RESULTS: Three independent reviewers abstracted test performance characteristics from each study for the performance of a 12-hour urine collection for the diagnosis of proteinuria defined as 300 mg in 24 hours. Diagnostic meta-analysis was performed to obtain summary statistics. Heterogeneity was assessed using the Cochrane Q or I2. Receiver operating characteristic curve analysis was used to assess the optimal diagnostic cutpoint for proteinuria from a 12-hour urine collection. Stratified analysis was performed based on whether patients were on bed rest during urine collection. A total of seven studies met inclusion criteria. The 12-hour urine protein was overall highly predictive of proteinuria on 24-hour urine collection area under receiver operating characteristic curve: 0.97 (95% confidence interval [CI] 0.95-0.98). The pooled sensitivity was 92% (95% CI 86-96) and specificity was 99% (95% CI 75-100). The optimal cutpoint based on the receiver operating characteristic curve was 150 mg of protein on 12-hour collection. CONCLUSION: A 12-hour urine collection compares favorably with a 24-hour urine collection for the diagnosis of proteinuria in women with suspected preeclampsia and has the advantage of convenience and improved clinical efficiency.