Serial histoplasma complement fixation is a test for antibodies to the fungus Histoplasma by a serologic technique called complement fixation. This test is usually repeated about 4 weeks later. If a patient has an active infection, the antibodies should increase over that time period. This is called a “rise in the titer of paired serologies”.
How the test is performed
Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.
A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
Infant or young child:
The area is cleaned with antiseptic and stuck with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test
There is no special preparation for the test. The test will be repeated in 10 days to 2 weeks or longer.
Infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following:
- Infant test or procedure preparation (birth to 1 year)
- Toddler test or procedure preparation (1 to 3 years)
- Preschooler test or procedure preparation (3 to 6 years)
- School age test or procedure preparation (6 to 12 years)
- Adolescent test or procedure preparation (12 to 18 years)
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
The test is performed to detect fungal infection caused by Histoplasma. A repeated test may show an increase in the titer, which would suggest an active rather than a prior infection.
Serology refers to the study of serum for its antibody content. An antibody defends the body against bacteria, viruses, fungus, or other foreign bodies (antigens). Certain microbial cells stimulate the body to produce these antibodies during active infection. Antibody production increases during the course of infection. In the initial stage of an illness, little Antibody may be detected. For this reason, serology tests are often repeated 2 to 4 weeks after the initial sample.
In the laboratory, the antibodies react with antigens in specific ways that can be used to confirm the identity of a specific microorganism. There are several serology techniques that can be used depending on the suspected antibodies. Serology techniques include agglutination, precipitation, complement-fixation, fluorescent antibodies, and others.
The presence of no antibodies is normal. People who have prior exposure to Histoplasma may have antibodies, often at low levels. Not everyone who has antibodies will have experienced a prior illness.
What abnormal results mean
Abnormal results show at least a 4-fold rise in antibody titers, suggesting active infection with histoplasmosis.
What the risks are
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate veins
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.
by Brenda A. Kuper, M.D.