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

Objective: To assess how urine concentration and biochemical assay influenc e the assessment of proteinuria. Methods: This was a prospective study to assess the accuracy of detection a nd quantification of proteinuria within the day assessment unit and antenat al ward of a teaching hospital in Leicester, United Kingdom. We studied hyp ertensive pregnancies (of mixed parity) referred to day care assessment or attending the antenatal hypertension clinic after 20 completed weeks of ges tation (n=197). Aliquots of a well-mixed 24-h urine collection were tested by routine dipstick urinalysis and then assayed for protein using the Benze thonium Chloride and the Bradford assays (n=197). Main Outcome Measures: Total protein excretion in 24 h and protein concentr ation per liter of urine for both biochemical assays were compared to semiq uantitative 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 pre valence (16.2%) and the Benzethonium Chloride assay measuring total protein excretion in 24 h recorded the highest (70.1%). When the positive and nega tive predictive values for dipstick urinalysis were calculated, performance was found to be dependent on both the units of measurement compared and th e type of assay used as the "gold standard." Positive predictive values ran ged 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 d ependent on the method used to detect it. The amount of protein assessed qu antitatively is further dependent on the biochemical assay employed. Howeve r, regardless of the quantitative assessment, dipstick urinalysis has a sig nificant false-negative rate. This first reporting of a variation in perfor mance between dipstick urinalysis and two different biochemical assays in p regnancy may be explained in relation to protein assay specificity and the observed protein compositions of the samples on electrophoretic analysis. T he significance of proteinuria should be considered in light of the method used to detect it, but, ultimately, it must be related to clinical outcome.