Figure 2
Clinical decision using total phenytoin or adjusted phenytoin compared to measured free phenytoin. (A) The data is derived from three-by-three contingency tables comparing grouping of PHTtotal and PHTfree into lower than therapeutic reference range (L), within therapeutic reference range (T), and higher than therapeutic reference range (H). For the three bar graphs on the left, PHTtotal would tend to lead to underdosing relative to the PHTfree. For the three bar graphs in the center, PHTtotal and PHTfree are concordant. For the three bar graphs on the left, PHTtotal would tend to lead to overdosing relative to PHTfree. For patients who had multiple phenytoin measurements, only the chronologically first set of concentrations available is plotted. The data is from 756 patients. (B) Similar design as in (A) except that PHTadj_free (using the Sheiner-Tozer equation) is compared to PHTfree. The data is from 756 patients