Unexpected random urinary protein:creatinine ratio results–limitations of the pyrocatechol violet-dye method
1 Department of Pathology and Laboratory Medicine, Children’s and Women’s Health Centre of British Columbia and the University of British Columbia, Vancouver, Canada
2 Department of Obstetrics and Gynaecology, University of British Columbia, Vancouver, Canada
3 Department of Medicine, Université de Sherbrooke, Sherbrooke, Canada
4 Department of Specialized Women’s Health, British Columbia Women’s Hospital and Health Centre, 4500 Oak Street, Room 1U59, Vancouver, BC V6H 3N1, Canada
5 Department of Medicine, University of British Columbia, Vancouver, Canada
6 Child and Family Research Institute, University of British Columbia, Vancouver, Canada
BMC Pregnancy and Childbirth 2013, 13:152 doi:10.1186/1471-2393-13-152Published: 17 July 2013
For clinicians, it is important to rely on accurate laboratory results for patient care and optimal use of health care resources. We sought to explore our observations that urine protein:creatinine ratios (PrCr) ≥30 mg/mmol are seen not infrequently associated with normal pregnancy outcome.
Urine samples were collected prospectively from 160 pregnant women attending high-risk maternity clinics at a tertiary care facility. Urinary protein was measured using a pyrocatechol violet assay and urinary creatinine by an enzymatic method on Vitros analysers. Maternal/perinatal outcomes were abstracted from hospital records.
91/233 (39.1%) samples had a PrCr ≥30 mg/mmol, especially when urinary creatinine concentration was <3 mM (94.1%) vs. ≥3 mM (16.4%) (p < 0.001). When using the last sample before delivery, 47/160 (29.4%) had a PrCr ≥30 mg/mmol in diluted urine vs. only 17/160 (15.4%) in more concentrated urine (p < 0.001); PrCr positive results were also more frequent among the 32 (20.0%) women with known normal pregnancy outcome (90.9% vs. 0) (p < 0.001). Using the same analyser, 0.12 g/L urinary protein was ‘detected’ in deionised water. Re-analysis of data from two cohorts revealed substantially less inflation of PrCr in dilute urine using a pyrogallol red assay.
Random urinary PrCr was overestimated in dilute urine when tested using a common pyrocatechol violet dye-based method. This effect was reduced in cohorts when pyrogallol red assays were used. False positive results can impact on diagnosis and patient care. This highlights the need for both clinical and laboratory quality improvement projects and standardization of laboratory protein measurement.