Renal tubular reabsorption rates, reabsorptive maxima, and apparent renal plasma excretory thresholds for carnitine were determined in four children with primary systemic carnitine deficiency (SCD), in two of the mothers of these children, in one patient with muscle carnitine deficiency (MCD), and in seven controls. In SCD the observed values were well below those found in six of seven controls, but one control, a healthy 20-year-old woman with normal muscle carnitine level, also exhibited a renal carnitine leak. In the two mothers of patients with SCD and in the case of MCD some of the parameters of the renal handling of carnitine were slightly altered. Tubular secretion of short-chain acylcarnitines was noted in patients and controls at high plasma free carnitine levels. Augmented excretion of short-chain acylcarnitines occurred at lower plasma free carnitine levels in the patients with SCD than in the other subjects. Free and short-chain acyl-carnitines may compete for the same renal reabsorptive site. A renal defect cannot fully account for primary SCD but may contribute to the carnitine depletion in this disorder.

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