Fractional tubular reabsorption (FTR) of free and acyl
carnitine was measured in 15 patients with various selective tubular transport defects and in 19 patients with more generalized tubular dysfunction (
Fanconi syndrome). FTR of free
carnitine was normal in all patients with a selective tubulopathy, FTR of acyl
carnitine was normal in most, and plasma
carnitine levels were normal without exception. In these patients, there was no evidence for the existence of a defective renal transport mechanism shared by
carnitine. In the patients with
Fanconi syndrome, mean FTR of free and acyl
carnitine was low; their plasma
carnitine levels were lowered and correlated with the FTR. In individual patients, FTR of free and acyl
carnitine also correlated with the severity of the disease. In the group of
Fanconi syndrome patients, FTR of free and acyl
carnitine correlated linearly with that of
valine. We concluded that the lowering of plasma
carnitine in the patients with
Fanconi syndrome was caused by excessive loss of
carnitine in urine. Its pathophysiological significance remained to be established.