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Interpretation of Amniotic Fluid White Blood Cell Count in “Bloody Tap” Amniocenteses in Women With Symptoms of Preterm Labor

OBJECTIVE: To estimate whether blood-contaminated amniotic fluid affects the performance of white blood cell (WBC) count in diagnosing intraamniotic inflammation and infection. METHODS: Three hundred fifty-seven consecutive women pregnant with singletons undergoing amniocentesis to rule out infection were enrolled prospectively. A “bloody tap” was defined as a red blood cell (RBC) count of 1,000 cells/mm3 or more. Proteomics analysis of amniotic fluid was used in this study as the standard for diagnosing inflammation. Infection was confirmed by positive amniotic fluid culture. An amniotic fluid WBC count correction formula was computed using maternal WBC count, hematocrit, and mean corpuscular volume. RESULTS: The prevalence of a bloody tap amniocentesis was 22% (77 of 357). In the absence of inflammation, the amniotic fluid WBC count was significantly higher in bloody tap (median [interquartile range] 18 [9–58] cells/mm3) compared with non–bloody tap specimens (4 [1–10] cells/mm3; P<.001). The correction formula reversed this difference to a nonsignificant level (bloody tap 0 [0–17] compared with non–bloody tap 3 [1–10] cells/mm3; P=.273). In the setting of inflammation, the observed WBC count of bloody tap samples (778 [197–2,062 cells/mm3]) was significantly elevated compared with that of the non–bloody tap specimens (616 [105–1,730] cells/mm3; P=.023). Correction of the WBC count in bloody tap amniocenteses improved the test accuracy and positive likelihood ratios for inflammation and infection. A correction algorithm was not useful in amniotic fluid specimens with less than 1,000/RBCs/mm3 or WBC counts more than 1,100 cells/mm3. Given the nonlinear relationship between amniotic fluid WBC and RBC, for a rapid correction of WBC count, the number of neutrophils that need to be subtracted from the observed WBC count is variable. CONCLUSION: In the setting of an amniotic fluid sample contaminated with 1,000 RBCs/mm3 or more, WBC count is a less accurate indicator of inflammation and infection. In such samples, correction of WBC count enhances diagnostic performance for inflammation and infection. LEVEL OF EVIDENCE: II