To the Editor.
—In the review article by Johnson and Rossof that appeared in the January Archives (1983;143:111-114) entitled "The Role of the Human Tumor Stem Cell Assay in Medical Oncology," the authors state that according to the table, the specificity of the assay is 0.12 and is determined as being the false-positives divided by the sum of true-positives and true-negatives. The specificity of an assay represents the ratio of true-negatives over the sum of true-negatives and false-positives. Accordingly, the specificity of the assay is 0.88 and not 0.12. The next statement: "Therefore, the assay is considerably better at identifying drugs to which the tumor is resistant than at identifying drugs to which the tumor is sensitive" does not reflect the interpretation of the result, since the assay is as much sensitive as specific, according to the data given in the table.I think it is useful to clarify this