Conversion factor:

• mg/L x 4.31 = micromol/L
• micromol/L x 0.232 = mg/L

After loading with 1000 mg phenobarbitone, the peak (200 micromol/L) and the 12 hour concentration (140 micromol/L) are plotted.

This is steeper than a line drawn from the peak concentration parallel to the bold lines (i.e. t1/2 < 50 h) and therefore supplementation is necessary.

Extrapolation predicts a concentration of 100 micromol/L at 24 hours. There are 4 spaces (between broken lines) between the t1/2 = 50 h line and the expected concentration at 24 hours. Therefore 4 x 5% of 1000 mg or 200 mg is required for supplementation at 24 hours to create an apparent half life of 50 hours.

Subsequent concentrations at 48 and 72 h of 80 and 60 micromol/L indicate no further supplementation is needed as they are above the ideal (t1/2 = 50 h) line.