One of the metabolites of procainamide (PA) is N-acetylprocainamide (NAPA). NAPA has some clinical activity but is more important in contributing to toxicity if it accumulates.
A patient who is a rapid acetylator will have a higher peak concentration of NAPA compared to a slow acetylator.
Measurement of NAPA is important in patients:
(1) with moderate to severe renal failure
(2) showing unexplained signs of procainamide-related toxicity
sum of procainamide and NAPA concentrations =
= (serum procainamide concentration in mg/L) + (serum NAPA concentration in mg/L)
Interpretation:
• A patient is at increased risk for showing signs of toxicity if the sum is > 25 mg/L.
• A patient may experience signs of toxicity at lower concentrations.
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