Correlation of free phenytoin to serum albumin in cancer patients. The objectives of this study were to compare the total and free phenytoin serum concentrations of cancer patients with hypoalbuminemia with those of cancer patients with normal serum albumin and to correlate the percentage of free phenytoin with the albumin concentration. A total of 22 patients were studied, 13 with normal albumin concentration and 9 with low albumin. The mean free phenytoin in the normal albumin group was 9.9 (+/- 1.3) percent and 17.6 (+/- 4.6) percent in the low albumin group. With the groups combined, the mean free phenytoin was 13.1 percent (range 8.3-22.2) with the albumin range of 20-45 g/L. There was a significant negative correlation (r = -0.9, p less than 0.001) between the percentage of free phenytoin and the measured serum albumin. In cancer patients, the serum albumin concentration appears to be the key factor that determines the percentage of free phenytoin. In cancer patients with low serum albumin concentration, the total and free phenytoin concentration should be measured for adequate assessment of phenytoin therapy. 