The present study establishes whether the pH of samples is crucial for measuring total carnitine in human plasma by tandem mass spectrometry and if it is necessary to neutralize the samples after alkaline hydrolysis of acylcarnitines. Free and total carnitine of ten plasma samples were measured by a radioenzymatic assay as a reference method, and forward there were analyzed by tandem mass spectrometry divided into two groups: treated with hydrochloric acid and without hydrochloric acid measuring each sample five times for both