Growth inhibitory effect of sodium azide in chemosensitivity assays

In vitro chemosensitivity assays based on colony counting are plagued by persistent incidence of false-negative results. To avoid serious predictive errors, some investigators have utilized positive controls (known toxic compounds) as a quality control measure. Sodium azide (NaN3) at 6 mg/ml failed to inhibit colony formation in 17/35 (49%) assays; while in the thymidine incorporation assay, sodium azide was effective in inhibiting 124/131 (95%) specimens. Positive control substances for use in chemosensitivity assays must be carefully selected to insure accurate results.