An elevated intracellular cystine content in leukocytes is diagnostic of cystinosis. It is also important to establish a baseline leukocyte cystine level before starting treatment. Cultured Fibroblasts Special circumstances sometimes make it easier to obtain cultured skin fibroblasts than peripheral leukocytes. Also, in unusual types of cystinosis the leukocyte cystine content may be equivocal and the diagnosis can be substantiated by measuring the cystine in cultured skin fibroblasts . If the infant has cystinosis, treatment can start before any symptoms occur.
Choronic villi sampling for prenatal diagnosis has the advantage of providing an earlier diagnosis.
In both procedures the cells are cultured and assayed for cystine, which is very elevated in cystinotic cells in both situations. Their leukocyte cystine content averages four to five times higher than normal. Family members are frequently anxious to know if they are carriers, especially when they marry and plan to start a family. The optimal way to determine this is to measure cystine in isolated polymorphonuclear leukocytes. Since that time, the Cystine Determination Laboratory has studied a few obligate heterozygotes whose leukocyte cystine values fell into the high normal range.