Cat scratch disease blood test, indirect fluorescent antibody test (IFA) for IgM and IgG against B. henselae
This blood test screens for exposure to Bartonella henselae, the bacteria that cause cat scratch disease. These bacteria are spread by cats, especially kittens, which are more likely to be infected. The disease is usually fairly mild and clears up without treatment, but it can be more serious for people with a weakened immune system.
If you are infected by B. henselae, your immune system may form proteins called antibodies to fight the infection. One way to diagnose cat scratch disease is to check your blood for these antibodies.
Your doctor may order this test if you have been scratched or bitten by a cat and have symptoms of cat scratch disease. Even if you have only been in close contact with a cat, you may have the test if you have symptoms of the disease. Common symptoms of cat scratch disease include:
Infected or swollen scratch or biteSkin rashSwollen lymph nodes around your head, neck, or armpitsFeverHeadacheTirednessLoss of appetite
In rare cases, your doctor may need to take a lymph node sample, or biopsy, to examine under a microscope for the bacteria that causes cat scratch disease.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Having a positive test for cat scratch antibodies does not always mean you have an active infection, but it does mean that you have been exposed to the bacteria. This test will look for antibodies called immunoglobulin G (IgG) and immunoglobulin M (IgM). These antibodies are measured in titers. You may have cat scratch disease if:
You have a single elevated titer
You have an IgG titer greater than 1:64
You have an IgG titer greater than 1:256, which suggests active or recent infection
You have a positive test for IgM, which suggests acute disease
You have a titer that rises significantly over time
Your doctor will decide if you need to be treated for cat scratch disease based on your blood test, any other diagnostic tests, and your symptoms.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries small risks that include bleeding, infection, bruising, and a sense of lightheadedness. When the needle pricks your arm, you may feel a slight sting or pain. Afterward, the site may be sore.
Finding antibodies to B. henselae is a reliable way of diagnosing cat scratch disease. But you may have symptoms before your antibodies get high enough to detect. You might need to have this blood test repeated over time.
You don't need to prepare for this test.