A clinical prediction rule has been developed for identifying acute and suba-cute HP that consists of six predictors:

1. Good clinical and occupational or environmental history of likely or known exposure to offending antigens, i.e., measurement of microorganisms in the relevant environment

2. Serum precipitating antibodies to thermophilic bacteria

3. Recurrent episodes of symptoms

4. Finding of rales on lung exam

5. Symptom onset 4 to 8 hours postexposure

Serum precipitins are present in about 90% in acute cases but tend to wane later in the course of disease. Serum precipitins have been described to specific agents causing HP. The majority of precipitins are IgG and IgA, but IgM precipitins have also been described (80-85). The prevalence of precipitins in exposed symptomatic and asymptomatic individuals varies depending on the number of antigens and how recent and severe the exposure to antigens. However, presence of serum precipitins does not always indicate disease. Asymptomatic exposed individuals can have positive serum precipitins with an estimated prevalence of 5% to 20%. Furthermore, absence of measurable serum precipitins does not rule out a diagnosis of HP because the correct antigen may not be available for testing, and the titer of antibodies can decline to nondetectable levels with chronic disease (86-88).

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