Screening for late preeclampsia at 35-37 weeks by the urinary Congo-red dot paper test.

Background: Several cross-sectional studies have investigated the incidence of urinary Congo-red dye positivity in women with preeclampsia (PE), compared to unaffected pregnancies, and reported very high sensitivity and low false positive rate in the diagnosis of PE.
Objective: To determine the performance of the urinary Congo-red dot paper test at 35-37 weeks' gestation in the prediction of delivery with PE at ≤2 and >2 weeks after assessment.
Methods: This was a prospective observational study in women attending for a routine hospital visit at 35 ⁺⁰ to 36 ⁺⁶ weeks' gestation in a maternity hospital in England. Urine samples were collected and the Congo-red dot paper test was used to assess the degree of Congo-red dye positivity. The test uses a scoring system from 1 to 8 and the higher the score the greater the degree of Congo-red dye positivity. We examined and compared the degree of Congo-red dye positivity in the groups that delivered with PE at ≤2 and >2 weeks with those that remained normotensive. Reproducibility was assessed by examining the inter- and intra-observer reliability of scoring on stored images with the researchers blinded to previous results.
Results: The study population of 2140 women included 46 (2.1%) that subsequently developed PE (2.1%). The urinary Congo-red dot test was positive in 8.3% (1/12) and 2.9% (1/34) that delivered with PE at ≤2 and >2 weeks from assessment and in 0.2% (4/2094) of the unaffected pregnancies when the cutoff for Congo-red dye positivity was ≥5. The respective values when the cutoff used was ≥3 were 66.7%, 23.5%, and 16.5%, respectively. The intraclass correlation coefficient for the inter-observer reliability was 0.926 (95% CI 0.890-0.953, p <.0001) and Cohen's kappa coefficient for the intra-observer reliability was 0.904, p <.0001.
Conclusions: The performance of the urinary Congo-red dot paper test at 35-37 weeks' gestation in the prediction of PE is very poor.