Physician Use of Parasite Tests in the United States from 1997 to 2006 and in a Utah CryptosporidiumOutbreak in 2007

ABSTRACTParasitic infection is uncommon in the United States, but surveys suggest that physicians test when the presence of parasites is unlikely and fail to order appropriate testing when suspicion is high. Numerous studies confirm that immunoassays are more sensitive for Giardiaand Cryptosporidiumdetection, but our experience was that physicians preferentially used ovum and parasite examination (O&P). We conducted a retrospective study of fecal parasite testing at a referral laboratory nationally (1997 to 2006) and during a Cryptosporidiumoutbreak (Utah, 2007) to correlate physician use of O&P and enzyme immunoassays (EIAs) with the yield of parasites detected. Nationally, of 170,671 episodes, 76.0% (n= 129,732) included O&P, 27.9% (n= 47,666) included GiardiaEIA, and 5.7% (n= 9,754) included CryptosporidiumEIA. Most pathogens were Giardiaor Cryptosporidium. More episodes were positive when EIA was performed (n= 1,860/54,483 [3.4%]) than when O&P only was performed (n= 1,667/116,188 [1.4%]; P< 0.001), and EIA was more sensitive than O&P. However, more O&P results were positive among patients with both O&P and EIA performed (2.5%) than among those with O&P only performed (1.4%; P< 0.001), suggesting that patients tested by O&P only may have been at lower risk. During the first 10 weeks of the outbreak, physicians also preferentially used O&P over EIA, but no Cryptosporidiumcases were detected by O&P. We conclude that clinicians frequently use O&P testing when test performance and epidemiology support the use of immunoassays or no testing. We recommend that stool O&P be limited to patients with negative immunoassay results and persistent symptoms or individuals at increased risk for non-Giardia, non-Cryptosporidiuminfection. An evidence-based algorithm for the evaluation of patients with suspected intestinal parasitic infection is proposed.