How Best to Use Histoplasma Antigen for Monitoring Therapy

Case Report

A 32-year-old woman who was receiving biological therapy for psoriasis presented with fever of 102.6°F, shortness of breath, 5% weight loss, and hepatomegaly. Histoplasmosis was suspected and urine and serum were submitted for Histoplasma antigen detection. Both results were above 19 ng/mL (above limit of quantification-ALQ). The urine remained ALQ for 6 weeks after initial testing; the serum declined to 3.1 ng/mL (figure).

Discussion

Often only the urine is tested for antigen and in cases where it is ALQ, it may remain ALQ more than 6-24 months after initiation of treatment, raising the question whether the treatment is working.

In most cases where the urine is ALQ, the serum is quantifiable initially or within a month of treatment [1] providing a better marker for assessing response. As the serum antigen becomes negative or low level (<2 ng/mL), the urine antigen is quantifiable and provides a marker to aid in decisions to stop treatment.

Recommendations

• Send both urine and serum for antigen testing to aid in diagnosis
• If the urine result is ALQ, monitoring the serum until it is negative or <2ng/mL
• Revert to testing urine once serum antigen is negative
• Monitor antigen:

3-month intervals during treatment

6 and 12 months after stopping treatment

Any time clinical or imaging findings suggest progression or relapse

Reference List

1. Hage CA, Kirsch EJ, Stump TE, et al. Histoplasma Antigen Clearance during Treatment of Histoplasmosis in Patients with AIDS Determined by a Quantitative Antigen Enzyme Immunoassay. Clin Vaccine Immunol 2011 Apr; 18(4):661-6.