Physician use of parasite tests in the United States from 1997 to 2006 and in a Utah Cryptosporidium outbreak in 2007

Abstract

Parasitic 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 Giardia and Cryptosporidium detection, 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 Cryptosporidium outbreak (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 Giardia EIA, and 5.7% (n = 9,754) included Cryptosporidium EIA. Most pathogens were Giardia or 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 Cryptosporidium cases 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-Cryptosporidium infection. An evidence-based algorithm for the evaluation of patients with suspected intestinal parasitic infection is proposed.

Fig. 1.

Proposed test algorithm for parasitic evaluation of patients with persistent diarrhea or gastrointestinal complaints. Other patient subgroups (e.g., patients with persistent eosinophilia, elevated IgE levels, urticaria, or other skin manifestations suggestive of parasitic infestation) may benefit from additional testing or referral to a specialist for complete evaluation. ^a, evaluation of patients for stool parasites is generally not recommended for illnesses of <7 days' duration (10, 23); ^b, may rarely include O&P or PCR for potential parasitic pathogens; ^c, prior residence or extensive travel in regions endemic for E. histolytica or Strongyloides stercoralis. Most cases of E. histolytica/E. dispar infection reported in the United States and Canada are due to nonpathogenic species (26). Outdated reports describe Strongyloides infection acquired in rural areas of Appalachia and the southeastern United States (21).