Can we defer/omit a type and screen blood test for pregnant women who know their blood type?

Background: Current clinical practice in many emergency departments (EDs) includes checking a type and screen blood test (T&S) for Rhesus (Rh) status on all pregnant patients presenting with vaginal bleeding or abdominal pain. The test is expensive, and awaiting results may delay disposition. Objective: To determine if there is a subset of pregnant women who reliably know their blood type and for whom a T&S blood test to determine Rh status can be safely omitted or deferred. Methods: A prospective study at two associated urban academic centers with an annual ED census of 150,000 patients was performed between January 2007 and June 2008. Pregnant patients who had a T&S obtained as part of their ED evaluation were enrolled. Subjects completed a standardized data form that requested demographic information and were asked to select "no," "maybe," or "yes, definitely" if they knew their blood type. Standard descriptive statistics with 95% confidence intervals were performed. Results: There were 319 pregnant women enrolled in the study. Among the 106 subjects that reported "yes, definitely" they knew their blood type, 103 (97.2%; 95% confidence interval [CI] 94.0100%]) identified their correct blood type and 105 (99.1%; 95% CI 97.2100%) identified their correct Rh status. None of these subjects selected a positive Rh when they were in fact a negative Rh. All 14 (13.2%) subjects with a negative Rh status identified themselves as having a negative Rh. Conclusion: Pregnant women reporting that "yes, definitely" they know their blood type, are reliable. Deferring T&S testing test may be reasonable.