Histoplasma complement fixation is a blood test that checks for signs of infection due to a fungus called Histoplasma capsulatum (H. capsulatum). Some people who breathe in particles of this fungus may get an infection called histoplasmosis.
A blood sample is needed. For information on how this is done, see: Venipuncture
The complement fixation test looks to see if the body has produced antibodies to a certain antigen (a substance that triggers an immune response) -- in this case, the Histoplasma capsulatum fungus. If antibodies are present, they attach to the antigen. This combination activates, or "fixes" complement, and this activation can be measured. That is why the test is called "complement fixation."
The test looks for antibodies in the clear liquid part of the blood (serum). The general term for this method is called serology.
How to prepare for the test
There is no special preparation for the test.
How the test will feel
You may feel a prick or stinging sensation when the needle is inserted to draw your blood. Some people may have moderate pain. Afterward, there may be some throbbing.
Why the test is performed
Histoplasma complement fixation is one of the most commonly used methods to test for a fungal infection caused by H. capsulatum.
The absence of antibodies (negative test) is normal.
What abnormal results mean
Abnormal results may mean you have an active histoplasmosis infection.
People who have been exposed to H. capsulatum in the past may also have antibodies to it, often at low levels. However, they may not have shown signs of illness.
Note: In the first stage of an illness, few antibodies may be detected. Antibody production increases during the course of an infection. For this reason, tests are often repeated several weeks after the first test.
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Deepe GS Jr. Histoplasma capsulatum. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 264.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.