Print out the nomogram. A worked example is here.

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

After phenobarbitone loading, plot the peak (2h) and a second concentration point (12-24h) against time.

If the slope between them is less steep than the solid lines (i.e. t1/2 > 50 h) no supplementation or further concentrations are required. If the slope is steeper, use the difference between the expected concentration (by extending the line between the measured concentrations to 24h) and the "ideal" concentration (obtained from a line drawn from the peak parallel to the t1/2 = 50h lines) to calculate the required supplement.

Each space (from dashed lines) between the predicted and ideal concentrations at 24h indicates 5% of the loading dose is required as a supplement for that 24 hour period.

Each subsequent day, concentrations can again be measured and compared with the "ideal" concentration obtained from the initial peak concentration, with supplementation given daily as necessary.
Repeat until concentrations are less than 30 micromol/L.

Note: if phenobarbitone supplements are required but not given the subsequent calculations of dose will not work