Metformin is an oral antihyperglycemic agent used in the therapy of noninsu
lin-dependent diabetic patients. This biguanide can induce dangerous compli
cations such as lactic acidosis when its plasma concentration is too high.
For this reason, the determination of plasma metformin should always be don
e during treatment. We developed a new HPLC method, for the routine determi
nation of plasma metformin, with good reliability, rapid execution, and low
costs. Sample preparation involved precipitation of the plasma proteins co
ntaining the internal standard buformin with a mixture of methanol, zinc su
lfate, and ethylene glycol: the diluted supernatant was injected into a cat
ion-exchange column. The mobile phase was potassium dihydrogenphosphate buf
fer-containing acetonitrile. The eluent was monitored at 236 nm. The calibr
ation curve is linear within the range of 20-4000 ng/mL; the within-day coe
fficients of variation were lass than 2.2% for metformin and 1.5% for bufor
min; the day-to-day coefficients of variation were less than 2.5% for metfo
rmin and 1.9% for buformin. The mean recoveries obtained from supplemented
samples were included between 99.4 and 104.2% for metformin, Many character
istics make this method useful and easily accessible to all clinical labora
tories equipped with HPLC instrumentation.