The true predictive value of the human tumor stem cell assay: does a workable assay select for treatment responders?

In practice, the human tumor clonogenic assay is workable for less than half of the patient population to which it is applied, since the remainder of the specimens fail to produce sufficient numbers of colonies. Thereby a bias may be introduced which could result in a false predictive value positive of the test. It is therefore necessary to compare the responses to treatment of patients whose tumors could be assayed in vitro to those whose tumors failed to grow adequately, to assure that the prevalence of treatment responders has not changed within the group of patients for which the assay worked. From an analysis of the treatment response of 70 patients with stage III and IV ovarian carcinomas and 70 patients with stage IV breast cancer, no selection bias did occur and no preferential in vitro growth of tumor samples from patients with treatment response was found.