Serologic Studies in Histoplasmosis

Significance of false-positive serologic tests for histoplasmosis and blastomycosis in an endemic area

False-positive serologic tests for histoplasmosis (H) and blastomycosis (B) are common in populations from endemic areas. In order to determine the significance of false-positive test results, we reviewed the final diagnoses of all patients whose sera were submitted to our laboratory for radioimmunoassay (RIA) and immunodiffusion (ID) during a 3-yr period. Of the 263 patients whose sera were examined, 29 (11%) had H or B; 41 (17.5%) of the remaining 234 patients had false-positive test results. Of these 41 patients, 31 were positive for H alone, and 10 had antibodies to both H and B. All three patients with false-positive ID tests for histoplasmosis also had positive titers (greater than or equal to 1:16) on RIA. No patient had a false-positive ID result for blastomycosis. The percentage of patients in each of five major diagnostic categories with and without false-positive serologic tests was similar (p greater than 0.05). The majority of patients had pulmonary infections, almost half of which were granulomatous infections other than H or B; this reflects the clinical indications for requesting fungal serologic tests. A positive fungal serology is not useful in suggesting the presence of a pulmonary disease other than H or B in patients from an endemic area suspected of having a pulmonary mycosis.

Reevaluation of diagnostic histoplasma serologies

There are few quantitative guidelines for the interpretation of serologic tests for histoplasmosis. Furthermore, guidelines that have been proposed are not accepted generally. This report reviews experiences with complement fixation and latex agglutination tests performed in a large acute-care hospital in an endemic area, and depicts the relationship between the proportion of false-positive reactions and the titer at which the serologic tests were reactive. We noted a considerable number of false-positive reactions for these widely-used tests. Use of a battery of standard tests for histoplasmosis resulted in a 90% decrease in false-positive reactions. Reliance upon multiple positive results in such a battery of tests would result in failure to detect 50% of cases of disseminated histoplasmosis, but would eliminate most false positives.

Evaluation of a microtiter latex agglutination test for histoplasmosis

Experiments were designed to evaluate a Microtiter latex agglutination (Micro-LA) test, as a serological aid in the diagnosis of histoplasmosis, and to compare this test with the conventional microtiter-complement fixation (CF) test for histoplasmosis. Sera tested were from cases of acute and chronic pulmonary and disseminated histoplasmosis, as well as from individuals not having histoplasmosis. Ninety-seven percent of the cases of acute pulmonary histoplasmosis had positive Micro-LA tests, whereas 91% had positive CF tests. Ninety-six percent of the patients having chronic pulmonary histoplasmosis showed positive Micro-LA tests and 91% had positive CF tests. In contrast, 64% of the cases of disseminated histoplasmosis had positive Micro-LA tests, whereas 82% had positive CF tests. None of these differences was statistically significant. Although there were no significant differences in complement fixing and agglutinating antibody cross-reactivity with Blastomyces antigens, more patients demonstrated CF titers than Micro-LA titers. Sera from patients with acute and chronic histoplasmosis showed higher Micro-LA titers than CF titers, whereas sera from cases of disseminated histoplasmosis showed higher CF titers. Histoplasmin skin testing has less of a boosting effect on agglutinating antibodies than on CF antibodies to histoplasmin. Anticomplementary sera can be used in the Micro-LA test. This test is simple to perform, and results can be obtained in 2 to 4 hr.

Benign histoplasmosis

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A simple method of preparing an antigen from Histoplasma capsulatum for use in the complement fixation test

A simple method for preparation of complement fixing antigen from Histoplasma capsulatum is outlined. The procedure consists of extraction of the sedimented yeast phase growth with pyridine for two hours and subsequent washing of the extracted sediment three times with buffered saline solution. The antigen so prepared is non-viable, potent, stable, and not anticomplementary; it appears to be suitable for use in routine diagnostic work.