Public Health Consequences of a False-Positive Laboratory Test Result for Brucella -- Florida, Georgia, and Michigan, 2005

A Pragle, MS; C Blackmore, DVM; TA Clark, MD; MD Ari, PhD; PP Wilkins, PhD; D Gross, DVM, PhD; EJ Stern, MD

Disclosures

Morbidity and Mortality Weekly Report. 2008;57(22):603-605. 

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Content

Human brucellosis, a nationally notifiable disease, is uncommon in the United States. Most human cases have occurred in returned travelers or immigrants from regions where brucellosis is endemic, or were acquired domestically from eating illegally imported, unpasteurized fresh* cheeses.[1,2] In January 2005, a woman aged 35 years who lived in Nassau County, Florida, received a diagnosis of brucellosis, based on results of a Brucella immunoglobulin M (IgM) enzyme immunoassay (EIA) performed in a commercial laboratory using analyte specific reagents (ASRs); this diagnosis prompted an investigation of dairy products in two other states. Subsequent confirmatory antibody testing by Brucella microagglutination test (BMAT) performed at CDC on the patient's serum was negative. The case did not meet the CDC/Council of State and Territorial Epidemiologists' (CSTE) definition for a probable or confirmed brucellosis case[3] ("CDC/Council of State and Territorial Epidemiologists Case Definition for Human Brucellosis For Public Health Surveillance"), and the initial EIA result was determined to be a false positive. This report summarizes the case history, laboratory findings, and public health investigations. CDC recommends that Brucella serology testing only be performed using tests cleared or approved by the Food and Drug Administration (FDA) or validated under the Clinical Laboratory Improvement Amendments (CLIA) and shown to reliably detect the presence of Brucella infection. Results from these tests should be considered supportive evidence for recent infection only and interpreted in the context of a clinically compatible illness and exposure history. EIA is not considered a confirmatory Brucella antibody test; positive screening test results should be confirmed by Brucella-specific agglutination (i.e., BMAT or standard tube agglutination test) methods.

On February 1, 2005, the Nassau County Health Department received a report, based on a positive Brucella antibody test result, of a possible case of brucellosis in a female resident aged 35 years. The woman reported having intermittent fever, chills, sweats, body aches, weakness, headaches, and malaise since October 13, 2004. She was examined first at a local emergency department in November 2004, diagnosed with acute bronchitis, and discharged without a prescription for antimicrobials. She subsequently was examined by an infectious disease specialist on January 3, 2005, at which time brucellosis was considered based on continued nonspecific symptoms, polyarthritis, and an atypical lymphocytosis seen on a peripheral blood smear. Blood was obtained for culture and Brucella antibody testing on January 3 and January 24. Blood cultures were negative after 10 days. The EIA results from a commercial laboratory were interpreted as positive IgM and negative IgG, consistent with early brucellosis. On the basis of these results, on February 1 the patient was prescribed a twice daily, 6-week course of antimicrobial therapy of doxycycline (100 mg) and rifampin (300 mg). She stopped taking both antimicrobials after a short period because of side effects. After confirmatory testing by BMAT of the January 24 serum sample, performed February 14 at CDC, was negative, no further treatment for brucellosis was recommended. No other infectious etiologies were identified as a cause of the patient's symptoms, and she was lost to follow-up.

After the February 1 report, but before receiving the negative BMAT results, Nassau County and Florida epidemiologists began an investigation to identify possible exposures to Brucella species. The patient reported no recent contact with animals or animal fluids. She had eaten goat cheese from several sources while traveling in Michigan during May-July 2004 and while staying at a Georgia youth hostel in July 2004, 3-5 months before her illness onset. No unpasteurized dairy products were discovered at the identified sources that supplied locations where the woman ate. At the youth hostel, all dairy products used for cooking were pasteurized and purchased from local markets; however, guests often contributed food they had brought with them, and exact origins of shared foods were difficult to determine.

*Fresh cheeses, such as cottage cheese and Neufchâtel, also are referred to as soft or unripened cheeses. They are made by curdling milk and draining the whey, with little additional processing, and spoil more quickly than processed hard cheeses.

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