The road to an FD diagnosis is often long and ambiguous. Many clinicians have never encountered this disease before. Below we describe the typical diagnostic path - step by step - to help you ask the right questions and know what to expect.
According to the FD/MAS International Consortium consensus, most FD/MAS diagnoses can be made after a complete clinical, radiological, endocrine, and dermatological evaluation. Biopsy is usually needed only in unusual or questionable cases, or when malignancy is suspected.
The exception, noted in the same guidelines (FD/MAS International Consortium, 2019): an isolated lesion in a single bone, without skin or endocrine features, can correspond to many different conditions - so it usually requires histological (biopsy) confirmation.
If biopsy is performed, however, GNAS mutation testing on biopsy material is in practice a must-have. Histopathology alone often cannot distinguish FD from other jaw fibro-osseous lesions - including ossifying fibroma (JOF/COF), which is treated differently. A GNAS mutation is a specific marker for FD; a Leiden review emphasizes it is absent in COF, COD, and low-grade osteosarcoma. A positive result strongly confirms FD. A negative result does not rule out disease entirely (mosaicism, small sample), but skipping the test when material is already available carries a high risk of misdiagnosis.
Sources: FD/MAS Consortium 2019 · Cleven et al. 2019 (PMC6968989) · PMC9832888
A dentist, primary-care physician, or parent notices the lesion. It may be a bone thickening, facial asymmetry, bone pain, or an incidental finding on an X-ray. Not every lesion hurts - many FD cases are discovered incidentally.
The clinician orders a computed tomography scan (CT) or CBCT (for jaw lesions). FD has a characteristic ground-glass appearance. MRI may be needed for soft-tissue assessment. In a typical clinical context, imaging can be sufficient for diagnosis, but it requires expertise in differentiating FD from other bone lesions.
Biopsy is not an automatic step for every suspected FD case. It is usually considered for an isolated monostotic lesion without other FD/MAS features, an atypical presentation, or red flags such as a soft-tissue mass, bone destruction, rapidly worsening pain, or night pain.
When biopsy material already exists, GNAS testing should be standard - especially for maxillofacial lesions. A Leiden UMC pathology review describes the GNAS mutation as a pathognomonic marker for FD among fibro-osseous lesions; detection rates range from 45-88% depending on the method. A positive result = FD. A negative result does not always exclude disease (mosaicism), but without the test FD is easily confused with JOF and unnecessary surgery may be planned.
Bone scintigraphy, whole-body MRI, or other whole-body imaging helps determine whether FD affects one bone or many. This matters for disease subtype and monitoring. In polyostotic disease or suspected MAS, the clinician should also assess skin and endocrine features.
Once the diagnosis is established: a monitoring plan based on lesion location, pain and fracture-risk assessment, endocrine evaluation, correction of deficiencies, and decisions about pharmacological treatment or surgery. In children without pain, progression, or functional impairment, observation is often preferred.
FD treatment options →Ask whether the clinical and radiological picture is typical for FD/MAS, whether the lesion is isolated, and whether there are red flags requiring biopsy: soft-tissue mass, bone destruction, rapid growth, or night pain.
Make sure the biopsy sample is preserved for molecular testing - ideally fresh or fresh-frozen, though GNAS testing can also be done from a paraffin block. If histopathology suggests FD, JOF, or another fibro-osseous lesion, ask for a GNAS mutation test on the same material.
Tell your doctor: "I would like to order a GNAS gene mutation test from the biopsy material, to confirm or rule out fibrous dysplasia and distinguish it from ossifying fibroma." This is not experimental - it is standard practice in differentiating jaw fibro-osseous lesions.
The material of choice is a sample from the lesion itself (a paraffin block after biopsy), as it gives the highest detection rate (about 80%). The mutation is detected from blood far less often (about 20-30%), because it is present in only some cells (mosaicism). And when the lesion is single and confined to one bone, e.g. in the craniofacial region, the mutation is usually absent from blood altogether. So a negative blood result excludes nothing, and is in fact expected, while a positive result confirms the disease.
This is one of the most common GNAS mutations in FD. Another frequently reported variant is c.601C>A (p.Arg201Ser). Both affect codon 201 of the GNAS gene and lead to constitutive activation of the Gs-alpha protein, resulting in abnormal bone development.
Below are four laboratories that publicly advertise testing for somatic GNAS mutations. The test is most often performed from affected bone tissue; some laboratories accept a paraffin block, and some also accept peripheral blood. Price, turnaround time, and ordering procedure can change - check current details on the chosen laboratory's website. The test usually requires a referral from the treating physician.
Somatic GNAS mutation testing via sequencing. Material: paraffin block. Turnaround: about 10 working days. This is where we ran our son's test - efficiently, with good service.
ul. Mogilska 86/3, Kraków · biuro@oncogene.pl · +48 12 410 58 73
Somatic GNAS mutation testing. Material: paraffin block. Turnaround: about 10 days. Sample drop-off and result pickup possible in Poznań (ul. Dąbrowskiego 77A) and Zielona Góra (ul. Towarowa 20).
cgm@genesis.pl · +48 61 62 63 436
A test within McCune-Albright syndrome diagnostics (includes GNAS mutation analysis). Performed by Warsaw molecular diagnostics provider Genomed. Check the website for current pricing and required documents.
Central Clinical Hospital of the Medical University of Łódź. Services of the Clinical Genetics Department and Outpatient Clinic, including selected molecular tests. Check the current price list and ordering path.
The list is not exhaustive. If you run or know of another Polish laboratory performing this test, write to us - we will gladly extend the list.
Javaid M.K., Boyce A., Appelman-Dijkstra N., …, Collins M.T. (2019). "Best practice management guidelines for fibrous dysplasia/McCune-Albright syndrome: a consensus statement from the FD/MAS international consortium", Orphanet Journal of Rare Diseases, 14(139). Open-access. A consensus of 51 experts from 13 countries; cited by Leiden, Sapienza, and UCSF as the foundation of contemporary clinical care. For diagnosis, it emphasizes that FD/MAS is most often diagnosed clinically after complete evaluation, with biopsy mainly reserved for unusual, questionable, or oncologically suspicious cases.
Open 2019 consensus (Orphanet J Rare Dis) →
Ferreira B.S.A., Cunha B.M., Moreira L.A., …, Valadares L.P. (2022). "Effects of zoledronic acid therapy in fibrous dysplasia of bone: a single-center experience", Archives of Endocrinology and Metabolism. In this cohort, some patients were diagnosed solely on typical radiological findings, consistent with the guideline approach.
Szymczuk V., Florenzano P., de Castro L.F., Collins M.T., Boyce A.M. (2024). "Fibrous Dysplasia / McCune-Albright Syndrome", GeneReviews® (University of Washington), updated February 2024. A clinical-genetics reference review by the NIH group that effectively defines FD/MAS practice. It confirms two points that matter for parents: diagnosis is most often based on typical clinical features, and the material of choice for GNAS testing is lesional tissue (about 80% sensitivity), not blood (about 20-30%, and usually negative in monostotic disease).
Open GeneReviews (NIH, 2024) →
Download flowcharts - care pathways (FD/MAS Alliance) →
Coming soon: a Polish version of the flowcharts - with ICD-10 codes, a list of Polish laboratories performing the GNAS test, and NFZ reimbursement pathways. This is the first concrete use case for our GeneGuidelines platform.