Friday, May 05, 2006

Diagnosing ITP

A low platelet count is an indication for treatment in CLL according to the NCI guidelines. However, this is only true if the cause is bone marrow suppression. There are two other fairly common causes in CLL: immune destruction (ITP) and hypersplenism (a big spleen).

The diagnosis of ITP is not easy. There is not a simple test like the Coombs test for AIHA that you can do. It is not easy to measue how long the platelets survive. Often the answer can only be determined by a bone marrow aspirate and biopsy. What you see in ITP are increased numbers of megakaryocytes (the cell that makes platelets). Thes often appear immature - their nuclei are not lobulated and there are no platelets budding off their cytoplasm.

This picture needs to be differentiated from MDS, another cause of thrombocytopenia, in which increased non-lobulating megakaryocytes may also be seen. It requires examination by an experienced pathologist who understands bone marrow - not someone whose day-to-day work is examining vasectomy specimens and breast cancer.


Vance Esler said...

I appreciate your comment about having a pathologist who knows how to read bone marrows (hematopathologist). It can be frustrating when you have one who primarily views solid tissue samples.

In the setting of CLL, both anemia and thrombocytopenia can be difficult to sort out, and a bone marrow exam is often the most direct approach. However, I do not routinely do bone marrow exams to diagnose ITP outside the setting of CLL. As you know, the diagnosis can often be made clinically. If the patient does not respond to simple measures, such as prednisone, then a further exam can be done.

So I was annoyed when I saw a young man, an ex-professional hockey player, who started bruising easily. He had a classic case of ITP. However, the primary doctor had ordered a peripheral smear to be reviewed by a pathologist. The pathologist, who is good with solid tumors, but not with blood, said in his report that he thought there might be lymphoblasts.

Well, there weren't, but once that label is on the chart, we have to prove it wrong. So the poor chap had to suffer through a bone marrow examination to prove that he only had what we already knew he had.

Picking the right pathologist is just as important a choice as finding the right clinicians. In the U.S., patients do not get to make this choice. The clinicians do not either, as the sample usually goes to whomever is next in rotation. Fortunately, most hematologists are adept at reviewing bone marrow samples themselves and are not completely dependent upon a pathologist.

Terry Hamblin said...


you are absolutely right. ITP is usually easy to diagnose clinically, but in the context of thrombicytopenia associated with CLL, a bone marrow is often the only way of knowing whether this is ITP, CLL, MDS or even hypoplastic anemia.

I always read my own bone marrows, but many young doctors aren't interested in learning this part of our trade.