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).
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  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.
- 3-month intervals during treatment
- 6 and 12 months after stopping treatment
- Any time clinical or imaging findings suggest progression or relapse