Tartrate resistant acid phosphatase (TRAP) is a test performed on blood cells or bone marrow (biopsy) to confirm a diagnosis of hairy cell leukemia. This test can also be done on blood plasma to look for signs of bone breakdown.

Type 5b acid phosphatase test; TRAP test

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

This test also can be done on a bone marrow biopsy.

No special preparation is needed for the blood test.

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.

Your doctor may order this test if you have signs of hairy cell leukemia. Hairy cell leukemia is a type of blood cancer in which TRAP levels are very high.

This test may also be done to monitor the activity of cancers that have spread to your bones. These cancers include multiple myeloma, and breast, lung, or prostate cancers. However, the test is not yet widely used for this purpose.

There should be less than 5 nanograms per milliliter (ng/mL) of TRAP.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

The example above shows the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

A high level of TRAP in leukemia cells supports a diagnosis of hairy cell leukemia.

A high level of TRAP in the blood plasma is a sign of bone breakdown. This can be due to many types of diseases, but in a patient with cancer it is likely a sign that the cancer has spread to bone.

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

This test is less often used to diagnose hairy cell leukemia because now there is a method to identify cells based on types of markers found on the cell surface (immunophenotyping).

Kantarjian H, O’Brien S. The chronic leukemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA: Saunders Elsevier; 2007:chap 195.