The rare, but potentially fatal, side effect of agranulocytosis has caused the use of clozapine to be regulated more than any other antipsychotic medication. All patients must be registered in the Clozapine National Registry and frequent monitoring of WBC (white blood cell) count and ANC (absolute neutrophil count) must be performed every 7-28 days depending on the current duration of treatment. According to the clozapine monitoring guidelines provided by the Clozapine National Registry, a WBC count less than 3000/mm³ or ANC less than 1500/mm³ requires interruption of clozapine therapy. However, some physicians and patients are reluctant to discontinue clozapine. There are limited published reports in which clozapine was continued and the neutropenia was managed by adding lithium or granulopoiesis stimulating factors (GSFs), changing concomitant medications that might also be implicated in neutropenia, or observing to see if neutropenia is transient.