Phenobarbital

Why?

Confirmation of suspected toxicity

Dose adjustment

LACK of other monitoring parameters

When?

Steady-state

What samples (sample times)?

As for digoxin

peak AND trough determinations are unnecessary at steady state. Some clinicians prefer peak, some prefer trough (always most consistent).

There is little variance between high and low at steady state.

Whichever is chosen, treat result as if it were an average concentrations.

Apply clearance-based formula to change dose rate.

How?

Same techniques as for gentamicin can be applied.

OR

Adjust single concentrations based on target and:

Therapeutic Concentration range 5 - 40 µg/ml

Toxic Concentration range 50 +, usually lethal @ 100 - 200

Examples

Good control after initial therapy

Determine concentration at steady state

(This concentration is effective)

9 months later, control "weakens"

If concentration lower than previous monitoring - increase dose according to digoxin methods

If concentration similar to previous monitoring - disease has progressed (control will require higher concentration).

Inadequate control after initial therapy

Determine concentration in patient

Select concentration in upper therapeutic range

Increase dose according to digoxin methods

Inadequate control with concentration high in therapeutic range

Phenobarbital anticonvulsant therapy alone is inadequate (switch drugs or try combination).