Implementation of an intraleukocitary cystine quantification method for diagnosis of cystinosis

Cystinosis is an inborn error of metabolism, clinically characterised by severe renal involvement and development of corneal cystine deposits, especially in the adult form of the disease. Cystinosis is a treatable condition. Therefore, an early diagnosis is necessary to start therapy. For biochemical confirmation of the condition it is necessary to quantify intracellular cystine concentrations. For this, different methods have been described with variations in cell isolation strategies and the amino acid quantification techniques used. In order to improve confirmatory biochemical diagnosis in our setting, a protocol for intraleukocitary cystine quantification was established.

A high performance liquid chromatography based method for cystine quantification in polymorphonuclear cells was implemented. Evaluation of the best anticoagulant to use and temperature stability of the sample at 4̊C were performed. In addition, we established reference values for our population.

It was determined that intraleukocitary cystine quantification must be performed in blood samples containing acid-citrate-dextrose as anticoagulant. Samples must be processed immediately due to their poor stability even when refrigerated. Based on the results from 50 healthy individuals, the cut-off point established for our population was 0.34nmol 1/2 cystine/mg.

The adaptation performed to the cystine quantification method here presented the highest control population that has been reported in the literature so far. Our results highlight the need for making available a cystine quantification method locally and confirm the convenience for each laboratory to establish its own reference values to provide greater reliability for interpreting results.