Historical control range

The number of revertant colonies on both negative (solvent or untreated) control plates should be in an historical control range described in literature or determined in the laboratory. 

To ensure that the results of an assay are valid, specific criteria have been determined. The frequency of micronucleated cells need to be within the normal historical control range. The frequency of micronucleated cells in the positive controls need to be significantly increased over the vehicle control. In addition, there must be five surviving animals per group. 

Male and female Fischer 344 rats exposed for 2 years to inhalation concentrations of 200, 600, and 1800 ppm tetrahydrofuran showed marginally increased incidences of renal tubule epithelial adenoma in male rats at the mid- and high- concentration exposures. The combined occurrence of renal tubule epithelial adenoma and carcinoma exhibited a positive trend. The incidence of combined tumors in mid and high dose males exceeded the historical control range. No tumors were observed in female rats. NTP concluded that tetrahydrofuran showed some evidence of carcinogenic activity in male rats but no evidence in female rats. The mode of action for the kidney tumors observed in male rats is via a nongenotoxic mechanism with associated increases in cell proliferation and evidence that supports a role of a-2U-globulin. 

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