In Cystinosis, cystine is stored inside cells in their lysosomes. The most convenient cells in which to measure cystine are leucocytes; white blood cells (WBCs). There is a reasonable assumption that WBC cystine broadly reflects cystine deposition in other tissues. Note that cystine is not raised in the red cells or plasma of patients with Cystinosis. Also that the urinary excretion of cystine is normal (Cystinosis must not be confused with Cystinuria).
WBC cystine measurement is central both to the diagnosis of Cystinosis and used for monitoring therapy.
WBC Cystine Expected Ranges
Note that the cystine concentration is expressed as “half cystine”. This is because in the assay cystine gets split into its two cysteine components and it is the total cysteine that is actually measured. Secondly note that the ½-cystine is expressed as a ratio of the total cellular protein in the sample.
- The normal range for WBC cystine is <0.2 µmol ½-cystine/g. In practice this is often below the range of detection.
- A WBC cystine >2 µmol ½-cystine /g supports a diagnosis of cystinosis. There is clear separation of normal range from that of affected untreated patients.
- The therapeutic target range for patients taking oral Cystagon™ is <1 µmol ½-cystine cystine/g protein.
When to take a WBC
Diagnosis Blood samples for WBC cystine can be taken at anytime.
Therapeutic monitoring of Cystagon™. Cystagon™ is required to be taken 6 hourly. Samples should be obtained 5-6 hours after the last dose of Cystagon™. This will show the WBC cystine at its high point before the next dose takes effect.
Haemodialysis does not affect the WBC cystine level.
For details of accredited laboratories in the UK click here