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Ask the Experts

Question:

How effective are blood tests in "catching" the serious side effects of Clozaril, such as agranulocytosis?

How often does stopping the clozaril result in reversing the side effects that were caused by Clozaril?

Is agranulocytosis the only thing that the blood tests screen for?

Answered by: L. Fredrick Jarskog

The blood test monitoring system for agranulocytosis for clozapine (Clozaril-brand name) is highly effective.

In the U.S. between 1997 to 2004, about 150,000 patient received Clozaril and 585 cases of agranulocytosis were identified in this group. That means that the rate of agranulocytosis in patients that take clozapine is about 5 in 1,000 (0.5%). Of the 585 cases, 19 were fatal. This means that the mortality risk associated with clozapine from agranulocytosis is about 1 in 10,000 (0.01%). In non-fatal cases, almost everyone experiences a full recovery from agranulocytosis once clozapine is stopped, allowing white blood cell production to resume.

The risk of agranulocytosis is highest during the first 6 months of treatment and white blood cell tests need to occur weekly during this period. During the second 6 months of treatment, monitoring frequency can be reduced to every 2 weeks, and after 1 year the frequency can drop to every 4 weeks, assuming that the white blood cell count has remained normal throughout this time.