title: Unexpected random urinary protein:creatinine ratio results-limitations of the pyrocatechol violet-dye method.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.forename: Dane A
authors.initials: DA
authors.lastname: De Silva
authors.forename: Anne C
authors.initials: AC
authors.lastname: Halstead
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Côté
authors.forename: Yasser
authors.initials: Y
authors.lastname: Sabr
authors.forename: Peter
authors.initials: P
authors.lastname: von Dadelszen
authors.forename: Laura A
authors.initials: LA
authors.lastname: Magee
mesh_terms: D000328:Adult; D001557:Benzenesulfonates; D015331:Cohort Studies; D004396:Coloring Agents; D003404:Creatinine / Q000652:urine*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D011225:Pre-Eclampsia / Q000175:diagnosis* / Q000652:urine; D011247:Pregnancy; D011256:Pregnancy Outcome; D011446:Prospective Studies; D011507:Proteinuria / Q000175:diagnosis*; D011748:Pyrogallol / Q000031:analogs & derivatives*; D016482:Urinalysis / Q000592:standards
chemical_list: D001557:Benzenesulfonates; D004396:Coloring Agents; D011748:Pyrogallol; C009134:pyrocatechol violet; C058731:pyrogallol sulfonphthalein; D003404:Creatinine