Effect of concentration and biochemical assay on the accuracy of urine dipsticks in hypertensive pregnancies.

OBJECTIVE: To assess how urine concentration and biochemical assay influence the assessment of proteinuria.

METHODS: This was a prospective study to assess the accuracy of detection and quantification of proteinuria within the day assessment unit and antenatal ward of a teaching hospital in Leicester, United Kingdom. We studied hypertensive pregnancies (of mixed parity) referred to day care assessment or attending the antenatal hypertension clinic after 20 completed weeks of gestation (n = 197). Aliquots of a well-mixed 24-h urine collection were tested by routine dipstick urinalysis and then assayed for protein using the Benzethonium Chloride and the Bradford assays (n = 197).

MAIN OUTCOME MEASURES: Total protein excretion in 24 h and protein concentration per liter of urine for both biochemical assays were compared to semiquantitative dipstick protein measurement.

RESULTS: The prevalence of proteinuria in the study group varied according to the method used for testing. Dipstick urinalysis recorded the lowest prevalence (16.2%) and the Benzethonium Chloride assay measuring total protein excretion in 24 h recorded the highest (70.1%). When the positive and negative predictive values for dipstick urinalysis were calculated, performance was found to be dependent on both the units of measurement compared and the type of assay used as the "gold standard." Positive predictive values ranged from 87.5% to 96.9% and negative predictive values ranged from 35.2% to 92.1%.

CONCLUSIONS: The prevalence of proteinuria in hypertensive pregnancies is dependent on the method used to detect it. The amount of protein assessed quantitatively is further dependent on the biochemical assay employed. However, regardless of the quantitative assessment, dipstick urinalysis has a significant false-negative rate. This first reporting of a variation in performance between dipstick urinalysis and two different biochemical assays in pregnancy may be explained in relation to protein assay specificity and the observed protein compositions of the samples on electrophoretic analysis. The significance of proteinuria should be considered in light of the method used to detect it, but, ultimately, it must be related to clinical outcome.