Skip to content
How It Works
Explore
Resources Ask a Question
Who It's For
Patients
Decode results, prep for appointments, weigh options.
Caregivers & advocates
Carry the research load for someone you love.
Providers & clinicians
Resources for patients in your practice.
Log In Ask a Question
PubMed This is a summary of 16 peer-reviewed journal articles Updated
Genetics

The Science of Discovery: Biology and Diagnosing Cystinosis

At a Glance

Cystinosis is diagnosed primarily through the white blood cell (WBC) cystine test, which measures trapped cystine levels. Doctors also use CTNS genetic testing and slit-lamp eye exams to confirm the disease and guide your child's ongoing treatment.

Understanding the biology of cystinosis helps explain why this condition affects so many different parts of the body. While the science can be complex, the core issue is a “transportation” problem at the cellular level. By identifying this problem through specific tests, your medical team can create a treatment plan tailored to your child [1][2].

The Biological Mechanism: A Broken “Exit Door”

Inside almost every cell in your child’s body are tiny compartments called lysosomes. Think of these as the cell’s recycling centers. Their job is to break down substances, like the amino acid cystine, so the body can reuse or dispose of them [3].

  • The CTNS Gene: This gene provides the blueprints for a protein called cystinosin [1].
  • The Role of Cystinosin: In a healthy cell, cystinosin acts like an exit door, allowing cystine to leave the recycling center and enter the rest of the cell [3][4].
  • The Mutation: In cystinosis, the CTNS gene has a “typo” (mutation). This means the cystinosin exit doors are either broken or missing entirely [5][6].
  • The Build-up: Because the cystine cannot exit, it gets trapped inside the lysosomes. Over time, it packs in so tightly that it forms hard, needle-like crystals [7]. These crystals eventually cause stress and damage to the cell, which is why organs like the kidneys and eyes are affected [6][8].

How Cystinosis is Diagnosed

Doctors use three primary “proofs” to confirm a diagnosis and determine the severity of the condition.

1. The WBC Cystine Test (The “Gold Standard”)

The most definitive way to diagnose cystinosis is by measuring the amount of cystine inside leukocytes (white blood cells) [2]. Since these cells travel throughout the body, they serve as a window into how much cystine is building up in other tissues.

  • Why it matters: This test not only confirms the diagnosis but is also used for the rest of your child’s life to check if their medication is working effectively [9][10].

2. Genetic Testing

By taking a blood or saliva sample, doctors can look directly at the CTNS gene [1].

  • What they look for: They are checking for two mutations (one from each parent). A common finding in families of Northern European descent is a specific “missing piece” of DNA known as the 57-kb deletion [1][11]. Knowing the specific mutation can sometimes help doctors predict how the disease might progress [12].

3. The Slit-Lamp Eye Exam

An ophthalmologist uses a specialized microscope called a slit-lamp to look at the front of the eye (the cornea) [13][1].

  • What they see: They are looking for tiny, shimmering crystals that are unique to cystinosis [14][15]. In young children, these are usually found in the front layers of the cornea, but they can move deeper as a child grows [14].

Lab Report Checklist

When you receive your child’s diagnostic reports, look for these specific terms and values:

Term to Look For What it Means Why it Matters
Leukocyte Cystine The “Gold Standard” test result [2]. Confirms the build-up of cystine in cells.
nmol 1/2 cystine/mg protein The standard unit of measurement. This is the “language” doctors use to track levels.
CTNS Mutation The specific genetic “typo” found [1]. Confirms the genetic cause and inheritance.
Biallelic Means two mutations were found. Confirms the child has the condition, not just a carrier.
Corneal Crystals Identified during the eye exam [13]. Confirms crystal accumulation in the eyes.

Note: In untreated patients, the leukocyte cystine level will be significantly elevated. The goal of treatment is to lower this number into a “target range” determined by your doctor [10][16].

Common questions in this guide

What is the gold standard test for diagnosing cystinosis?
The most definitive way to diagnose cystinosis is the white blood cell (WBC) cystine test. This blood test measures the amount of cystine trapped inside the white blood cells and is also used to monitor how well medication is working.
What causes the build-up of cystine in the body?
Cystinosis is caused by a mutation in the CTNS gene. This genetic typo creates a broken or missing cystinosin protein, which normally allows the amino acid cystine to exit cell recycling centers called lysosomes.
Why does my child need a slit-lamp eye exam for cystinosis?
An eye doctor uses a special microscope called a slit-lamp to look for tiny, shimmering cystine crystals in the front of the eye (the cornea). These crystals are unique to cystinosis and help confirm the diagnosis.
What does a biallelic CTNS mutation mean on a lab report?
Finding a biallelic mutation means that two genetic mutations were discovered, one inherited from each parent. This confirms that the child actually has cystinosis, rather than just being a carrier of the condition.
What is the 57-kb deletion in cystinosis?
The 57-kb deletion is a specific missing piece of DNA in the CTNS gene. It is a very common genetic mutation found in families of Northern European descent who are diagnosed with cystinosis.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is my child's current WBC cystine level, and is it measured in 'nmol half-cystine per mg protein'?
  2. 2.Does the genetic report show two mutations in the CTNS gene, and what do these specific mutations tell us about the type of cystinosis?
  3. 3.Did the ophthalmologist see crystals in all layers of the cornea, or just the front (anterior) part?
  4. 4.How often will we need to repeat the WBC cystine test to ensure the medication is working?
  5. 5.Can you explain the '57-kb deletion' if it appeared on our genetic report?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (16)
  1. 1

    CTNS mRNA molecular analysis revealed a novel mutation in a child with infantile nephropathic cystinosis: a case report.

    Papizh S, Serzhanova V, Filatova A, et al.

    BMC nephrology 2019; (20(1)):400 doi:10.1186/s12882-019-1589-2.

    PMID: 31672123
  2. 2

    The Pitfall of White Blood Cell Cystine Measurement to Diagnose Juvenile Cystinosis.

    Bondue T, Kouraich A, Berlingerio SP, et al.

    International journal of molecular sciences 2023; (24(2)) doi:10.3390/ijms24021253.

    PMID: 36674769
  3. 3

    Structural basis for proton coupled cystine transport by cystinosin.

    Löbel M, Salphati SP, El Omari K, et al.

    Nature communications 2022; (13(1)):4845 doi:10.1038/s41467-022-32589-2.

    PMID: 35977944
  4. 4

    JIP4 deficiency causes a lysosomal storage disease arising from impaired cystine efflux.

    Nassar LM, Shi X, Roczniak-Ferguson A, et al.

    bioRxiv : the preprint server for biology 2025; doi:10.1101/2025.06.06.657909.

    PMID: 40661466
  5. 5

    Computational prediction of deleterious nonsynonymous SNPs in the CTNS gene: implications for cystinosis.

    Adda Neggaz L, Dahmani AC, Derriche I, et al.

    BMC genomic data 2025; (26(1)):35 doi:10.1186/s12863-025-01325-2.

    PMID: 40375073
  6. 6

    Lysosomal cystine accumulation promotes mitochondrial depolarization and induction of redox-sensitive genes in human kidney proximal tubular cells.

    Sumayao R, McEvoy B, Newsholme P, McMorrow T

    The Journal of physiology 2016; (594(12)):3353-70 doi:10.1113/JP271858.

    PMID: 26915455
  7. 7

    Nephropathic cystinosis: an update on genetic conditioning.

    Topaloglu R

    Pediatric nephrology (Berlin, Germany) 2021; (36(6)):1347-1352 doi:10.1007/s00467-020-04638-9.

    PMID: 32564281
  8. 8

    Mitochondrial Dynamics of Proximal Tubular Epithelial Cells in Nephropathic Cystinosis.

    De Rasmo D, Signorile A, De Leo E, et al.

    International journal of molecular sciences 2019; (21(1)) doi:10.3390/ijms21010192.

    PMID: 31888107
  9. 9

    An international cohort study spanning five decades assessed outcomes of nephropathic cystinosis.

    Emma F, Hoff WV, Hohenfellner K, et al.

    Kidney international 2021; (100(5)):1112-1123 doi:10.1016/j.kint.2021.06.019.

    PMID: 34237326
  10. 10

    A novel sustained-release cysteamine bitartrate formulation for the treatment of cystinosis: Pharmacokinetics and safety in healthy male volunteers.

    Berends CL, Pagan L, van Esdonk MJ, et al.

    Pharmacology research & perspectives 2021; (9(2)):e00739 doi:10.1002/prp2.739.

    PMID: 33764642
  11. 11

    Deficiency of the sedoheptulose kinase (Shpk) does not alter the ability of hematopoietic stem cells to rescue cystinosis in the mouse model.

    Goodman S, Khan M, Sharma J, et al.

    Molecular genetics and metabolism 2021; (134(4)):309-316 doi:10.1016/j.ymgme.2021.11.006.

    PMID: 34823997
  12. 12

    Phenotypic variability in cystinosis: Lessons from an atypical case.

    Toso D, Furlano M, Tinoco A, et al.

    Nefrologia 2025; (45(7)):501342 doi:10.1016/j.nefroe.2025.501342.

    PMID: 41005824
  13. 13

    Diagnosis of Nephropathic Cystinosis in a Child During Routine Eye Exam.

    Ecel M, Sarı A, Delibaş A

    Turkish journal of ophthalmology 2017; (47(5)):292-295 doi:10.4274/tjo.69922.

    PMID: 29109899
  14. 14

    Examination of corneal deposits in nephropathic cystinosis using in vivo confocal microscopy and anterior segment optical coherence tomography: an age-dependent cross sectional study.

    Csorba A, Maka E, Maneschg OA, et al.

    BMC ophthalmology 2020; (20(1)):73 doi:10.1186/s12886-020-01336-w.

    PMID: 32102651
  15. 15

    Introducing the University of California, Irvine Corneal Cystine Crystal Score: A Novel Tool for Assessing Corneal Crystal Deposition in Cystinosis Patients.

    Rezaei K, Shahraki K, Chen L, et al.

    Scientific reports 2025; (15(1)):36262 doi:10.1038/s41598-025-20108-4.

    PMID: 41102237
  16. 16

    Therapeutic strategies in cystinosis: A focus on cysteamine and beyond.

    Santoro A, Ferrara YV, De Angelis A

    Experimental and molecular pathology 2025; (144()):104995 doi:10.1016/j.yexmp.2025.104995.

    PMID: 40912033

This page provides educational information about cystinosis biology and diagnostic tests. Always consult your child's geneticist or pediatric specialist for medical advice and test interpretation.

Get notified when new evidence is published on Cystinosis.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.