Tracking cystine levels

Blood tests measure the amount of cystine in white blood cells

The secret to tracking how well cystine-depleting therapy (CDT) is working is regular cystine level testing.

What is a cystine level test?

Cystine level testing measures the amount of cystine in white blood cells
(WBCs). Along with carefully following a treatment plan, routine testing is key. Testing is the only way to know a person’s cystine level.

Why cystine level testing is important

Tracking cystine levels over time helps people and their doctors know how well CDT is working. Knowing a person’s cystine level will help a doctor decide if a dose needs to be adjusted. Having the correct dose is the only way to help delay or limit damage to the body.

The cystine level testing schedule

The cystine level testing schedule

Two cystine tests. Two target levels.

There are two types of cystine level tests. They measure cystine levels differently.

Type of WBC cystine level tests Mixed leukocytes test Granulocytes test
Target cystine level range Below 1.0 nmol ½ cystine/mg protein Below 1.9 nmol ½ cystine/mg protein
Testing institution Baylor Genetics University of California San Diego (UCSD)
When to get the test The doctor will order WBC cystine level testing at the same point in the CDT dosing schedule.

It's important to work with a doctor to decide which cystine level test is right for you. Always know which test is being taken: Results mean different things with different tests. The same test should be taken each time to help compare test results over time.

Timing matters

In people with cystinosis, cystine levels usually change based on the time of the last dose of cystine-depleting therapy (CDT). It's important to have cystine levels tested at the right time to get useful test results. Ask your doctor what time of day to get your cystine levels tested.

To get the most out of treatment, people with cystinosis need to take their CDT regularly. It's very important to take the medicine at the same time every day and every night. Even the slightest delay may cause cystine levels to rise.


WBC Cystine Level Collection Kit

Testing kits are being offered at no cost. Plus, there is an instructional video to show doctors, patients, and lab employees the correct way to use the kit.

Learn more



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