SAN FRANCISCO — Getting a protein-to-creatinine ratio was more helpful than using a urine dipstick to measure proteinuria in patients with suspected preeclampsia, according to a retrospective cohort study.
The protein to creatinine (P/C) ratio correlated strongly with a 24-hour urine protein measurement, which is the standard for quantifying protein. The P/C ratio had a 90% correlation with 24-hour urine protein measurements, compared with only a 58% correlation between the urine dipstick and 24-hour urine protein measurements, Jasmine Lai and associates reported in a poster presentation at the annual meeting of the Society for Maternal-Fetal Medicine.
Investigators analyzed data on 140 women with suspected preeclampsia who had both a dipstick and 24-hour protein measurement, 177 who had both a P/C ratio and 24-hour protein measurement, and 244 who had both a dipstick and P/C ratio. The different assays were performed within 48 hours of each other for each patient.
The P/C ratio was a more sensitive marker for proteinuria, with a sensitivity of 75%, compared with dipstick's sensitivity of 44%, reported Ms. Lai, a student at the University of California, San Francisco, who conducted the study while a summer fellow at the University of California, San Diego.
Now that getting a dipstick measurement has been encumbered by the Clinical Laboratory Improvement Amendments law, it's just as fast and efficient to get a P/C ratio, Dr. Douglas Woelkers, the primary investigator in the study, said in an interview.
“Nurses now can't do dipsticks in the [labor and delivery] setting. Our hospital requires that all dipsticks go down to the laboratory to be read by machine. Why not get the more accurate P/C ratio, because it takes the same time to get a result back, and it's the same expense compared with the dipstick?” said Dr. Woelkers of the University of California, San Diego.
The dipstick underestimated proteinuria 44%–48% of the time, he added. Patients with a false-negative dipstick and mild hypertension would be sent home “only to find out later on that they truly had the disease and we weren't intervening soon enough,” he said.
Dipstick measurements were significantly confounded by the method of collection and the presence of blood, squamous cells, white blood cells, or leukocyte esterase, the investigators found.
To confirm the superiority of the P/C ratio, the investigators analyzed adverse maternal or fetal outcomes from delivery records of 209 patients who had both a urine dipstick and a P/C ratio. They used a composite of three or more markers for severe disease, including thrombocytopenia, elevated liver function tests, high creatinine level, low Apgar score, low birth weight, and maternal hospitalization longer than 3 days.
The P/C ratio was more accurate than the dipstick in predicting adverse outcomes because it more accurately measured proteinuria, Dr. Woelkers said. The P/C ratio had a sensitivity of 50% for composite adverse outcomes and a specificity of 72%. Taking one or more dipstick measurements was 35% sensitive and 81% specific for predicting adverse outcomes. Taking two or more dipstick measurements was 24% sensitive and 83% specific for adverse outcomes. Dr. Woelkers' hospital has converted entirely to doing P/C ratios instead of dipsticks for patients with suspected preeclampsia.
Further research will be needed to see if it makes sense to switch from dipsticks to P/C ratios for patients with suspected preeclampsia in office settings, not just in hospitals, he added.