Improved Diagnosis of Acute Pulmonary Histoplasmosis by Combining Antigen and Antibody Detection

Abstract

Background: Acute pulmonary histoplasmosis can be severe, especially following heavy inoculum exposure. Rapid diagnosis is critical and often possible by detection of antigen, but this test may be falsely negative in 17% of such cases. Antibody detection by enzyme immunoassay (EIA) may increase sensitivity and permit the measurement of immunoglobulin M (IgM) and immunoglobulin G (IgG) classes of antibodies separately.

Methods: Microplates coated with Histoplasma antigen were used for testing of serum from patients with acute pulmonary histoplasmosis and controls in the MVista Histoplasma antibody EIA. Results for IgG and IgM were reported independently.

Results: IgG antibodies were detected in 87.5%, IgM antibodies in 67.5%, and IgG and/or IgM antibodies in 88.8% of patients with acute pulmonary histoplasmosis in this assay, while immunodiffusion, complement fixation, and antigen testing showed sensitivities of 55.0%, 73.1%, and 67.5%, respectively (n = 80). Combining antigen and antibody detection increased the sensitivity to 96.3%.

Conclusions: The MVista Histoplasma antibody EIA offers increased sensitivity over current antibody tests while also allowing separate detection of IgG and IgM antibodies and complementing antigen detection. Combining antigen and EIA antibody testing provides an optimal method for diagnosis of acute pulmonary histoplasmosis.

Keywords: Histoplasma capsulatum; acute pulmonary histoplasmosis; serology.

Figure 1.

Receiver operating characteristic (ROC) curve for determination of anti-Histoplasma immunoglobulin G (IgG) and immunoglobulin M (IgM) antibody cutoff. The ROC-recommended cutoff optical density (OD) was 0.200 for IgG (assigned to 10 enzyme immunoassay [EIA] units [EU]). At this cutoff, sensitivity for IgG was 87.5% (n = 80) and specificity 95.0% (n = 198), area under the curve (AUC) was 0.959 (95% confidence interval [CI], .929–.979), and the standard error was 0.0132 (P < .0001). The ROC-recommended cutoff OD was 0.253 for IgM (assigned to 10 EU). At this cutoff, sensitivity for IgM was 70.3% (n = 80) and specificity 97.0% (n = 198), AUC was 0.879 (95% CI, .833–.916), and the standard error was 0.037 (P < .0001).

Figure 2.

Immunoglobulin G (IgG) and immunoglobulin M (IgM) response in MVista Histoplasma antibody enzyme immunoassay. Antibody levels in patients with histoplasmosis (Hc) (n = 80), blastomycosis (Bd) (n = 25), or coccidioidomycosis (Ci) (n = 25), and healthy patients or clinical controls (n = 198). The cutoff for positivity (10 units) is indicated by the broken horizontal line and the numbers below the broken line represent the number of patients with negative results.

Figure 3.

Reproducibility by linear regression. Interassay agreement of immunoglobulin G (IgG) unit values obtained from repeat testing of histoplasmosis patient samples showed strong correlation with a coefficient of determination (R²) of 0.978, a residual standard deviation of 3.638, a 95% slope confidence interval (CI) of .949–1.032, and a slope P value of <.01. Interassay agreement of immunoglobulin M (IgM) unit values obtained from repeat testing of histoplasmosis patient samples showed strong correlation with a R² of 0.994, a residual standard deviation of 2.235, a 95% slope CI of 1.008–1.021, and a slope P value of <.01 (Passing and Bablok method).