IgM and IgG antibodies to Histoplasma antigen usually appear during the first month of infection. The IgM antibody response in acute pulmonary histoplasmosis is detectable during the acute phase (roughly 3 weeks) and is shown to decline during the convalescent stage (at about 6 weeks); whereas, IgG levels remained relatively constant at 6 weeks. Follow-up testing may be considered 2-4 weeks after initial testing to determine if antibody levels are increasing, especially in patients with low positive (10 EU – 20 EU) or intermediate results. Increase in IgG antibody or decrease in IgM concentration would suggest recent infection. Antibodies may persist for several years in patients with chronic pulmonary complications or progressive extrapulmonary (disseminated) histoplasmosis. Antibodies may be falsely-negative in some progressive or chronic cases, especially in immunocompromised patients. Antibodies may also be detected in healthy subjects who are asymptomatic as a result of sub-clinical infection within the last 18-36 months.