TRAP - Tartrate Resistant Acid Phosphatase

Alternative names 
Type 5 acid phosphatase

A test of blood cells or bone marrow (biopsy) conducted to support a diagnosis of hairy cell leukemia. This test can also be done on blood plasma, in which case it is used as a marker of bone breakdown, for example to measure bone destruction caused by cancer.

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 an elastic band is placed around the upper arm to cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in air-tight vials 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.

Alternatively, the test can be done on a bone marrow biopsy. This test is discussed in detail in another section.

How to prepare for the test
No special preparation is necessary for the blood test.

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

Tartrate-resistant acid phosphatase (TRAP) is an isoenzyme of acid phosphatase which is found mainly in bone and some blood cells. Isoenzymes are different forms of an enzyme with different physical structures, but which catalyze (speed up) similar biochemical reactions. Isoenzymes frequently differ in concentration in different tissues.

hairy cell leukemia is a type of blood cancer in which TRAP levels have been found to be very high. A stain for TRAP on Leukemia cells in the blood or bone marrow can help support a diagnosis of hairy cell leukemia. If this test is negative, the diagnosis of hairy cell leukemia is unlikely.

TRAP is also released during the activity of osteoclasts, a type of cell that breaks down bone. Because of this, any condition in which bone is broken down faster than usual may lead to excessive levels of TRAP in the blood. In this case the TRAP is not contained within the blood cells, but is free in the plasma, the liquid part of blood.

The activity of cancers that invade bone, such as multiple myeloma, and breast, lung or Prostate cancer which has metastasized to bone, can be followed by measuring TRAP in the plasma. This test is not yet in widespread use for these conditions, but it may be used more in the future if research proves it to be useful.

Normal Values
There should be a decrease of acid phosphatase of less than 5 ng/ml in the presence of tartrate.

Note: ng/ml = nanogram per milliliter

What abnormal results mean

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

A high level in the plasma indicates bone breakdown. This can be due to many types of diseases, but in a patient with cancer it is likely related to bone invasion (metastasis) by the cancer.

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

Special considerations
This test has become less important in diagnosis of hairy cell leukemia since the widespread use of immunophenotyping, a method of identifying cells based on molecules found on the cell surface.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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