Doctors are discovering that unexplained bone loss and fractures in otherwise healthy adults can be the first sign of systemic mastocytosis, a rare immune disorder where mast cells accumulate abnormally in the bone marrow and other organs. A new case series published in Biomolecules reveals how four patients initially diagnosed with idiopathic osteoporosis (bone loss with no clear cause) were ultimately found to have this condition, highlighting a critical diagnostic gap in how secondary osteoporosis is evaluated.

What Is Systemic Mastocytosis and Why Does It Affect Bones?

Systemic mastocytosis (SM) is a clonal mast cell disorder, meaning abnormal mast cells accumulate in the bone marrow and other organs. Mast cells are immune cells that release inflammatory chemicals called cytokines when activated. This chronic inflammation can damage bone tissue, leading to osteoporosis and fractures even in younger, otherwise healthy people.

The condition is rare, but its symptoms can be subtle and easily mistaken for other diseases. In the case series, four adults were referred to endocrinology specialists for low-trauma fractures or unexplained bone loss. All four were ultimately diagnosed with indolent SM, the slowest-progressing form of the disease. Three of the four patients had initially been diagnosed with idiopathic osteoporosis, meaning doctors had found no secondary cause for their bone loss.

How Did Doctors Finally Identify the Real Problem?

The breakthrough came through targeted blood testing. All four patients had elevated basal serum tryptase levels, a marker of mast cell activity. Tryptase is an enzyme released by mast cells, and high levels in the blood suggest abnormal mast cell burden. Bone marrow biopsies then identified the KIT D816V genetic variant and atypical mast cells in each patient.

The clinical presentations varied. One 55-year-old man had a single low-energy fracture of the L5 vertebra. A 71-year-old woman with primary hyperparathyroidism experienced multiple vertebral compression fractures despite being on denosumab, a medication designed to prevent bone loss. A 43-year-old man presented with acute anaphylaxis triggered by a wasp sting, a classic sign of mast cell activation. The fourth patient had concurrent myelofibrosis, a blood disorder, but osteoporosis was his only clinical sign of SM.

Notably, none of these patients had skin lesions, which are sometimes associated with SM. This underscores how the disease can hide in plain sight, presenting only as bone fragility.

What Diagnostic Steps Should Doctors Consider?

The case series authors proposed a new diagnostic approach for adults presenting with unexplained or treatment-refractory osteoporosis. They recommend combining two key tests:

• Basal Serum Tryptase Assessment: A simple blood test measuring baseline tryptase levels, which rise when mast cells are abnormally abundant in the body.
• High-Sensitivity Peripheral Blood KIT D816V Testing: A genetic test detecting the specific mutation found in SM mast cells, providing confirmation of the diagnosis.
• Bone Marrow Biopsy: When tryptase and KIT testing suggest SM, bone marrow examination can identify atypical mast cells and confirm the diagnosis according to World Health Organization criteria.

These diagnostic steps are especially important in younger men, patients with a history of severe anaphylaxis, and those who fracture despite taking antiresorptive medications like bisphosphonates or denosumab.

What Treatment Options Are Available?

All four patients in the case series received intravenous zoledronic acid, a potent bisphosphonate that slows bone loss, along with vitamin D repletion to support bone health. KIT-targeted therapy, which blocks the abnormal signaling driving mast cell accumulation, is being considered for these patients as a longer-term strategy.

Beyond bone-specific treatment, managing SM requires controlling mast cell activation and the chronic inflammation it triggers. This is where understanding the inflammatory protein signature of the disease becomes valuable.

Can Blood Proteins Help Predict Disease Severity?

A separate study presented at the European Hematology Association 2026 Congress found that a circulating protein signature identified by machine learning can distinguish advanced from non-advanced SM with greater accuracy than conventional clinical parameters alone. Researchers profiled plasma from 81 adults with SM using a 250-protein inflammation panel and trained machine learning models to identify key biomarkers.

The proteomic model achieved 85.4% accuracy in distinguishing advanced from non-advanced disease, significantly outperforming a clinical model that achieved 75.8% accuracy. When proteomics and clinical features were combined, performance improved further, suggesting that inflammatory protein signatures could enhance risk stratification and disease monitoring.

The identified protein signature included several biologically relevant markers:

• IL1R1: A receptor involved in inflammatory signaling, indicating how the immune system's inflammatory response drives disease progression.
• FASLG: A protein involved in apoptotic pathways, suggesting abnormal cell death and survival mechanisms in SM.
• LDH: An enzyme reflecting metabolic activity and tissue damage, indicating the burden of disease on organs.
• VCAM1: A marker of endothelial activation, showing how SM affects blood vessel function and inflammation.
• IL1RL1: Another inflammatory signaling protein, reinforcing the role of cytokine-driven inflammation in SM pathophysiology.

"High-throughput inflammatory proteomics integrated with machine learning models identifies a stable circulating protein signature that distinguishes advanced from non-advanced SM beyond conventional clinical features, supporting development of inflammatory proteomic biomarkers for risk stratification and disease monitoring in systemic mastocytosis," concluded the researchers.

Study authors, presented at EHA 2026 Congress

How Should Patients and Doctors Respond to These Findings?

The case series authors emphasized that their findings represent diagnostic approaches to unexplained osteoporosis rather than general screening recommendations. However, they highlight an important clinical lesson: when a patient presents with bone loss that doesn't fit the typical pattern, secondary causes like SM should be investigated.

For patients with unexplained fractures or osteoporosis, especially those who don't respond well to standard bone-loss medications, asking a doctor about SM screening could be worthwhile. A simple tryptase blood test is a low-risk first step that could reveal a treatable underlying condition.

For clinicians, these findings suggest that the current approach to evaluating secondary osteoporosis may be incomplete. Adding tryptase testing and KIT mutation screening to the diagnostic workup for treatment-refractory bone loss could uncover cases of SM that would otherwise remain hidden, allowing earlier intervention and better long-term outcomes.

As research continues to refine both diagnostic methods and inflammatory biomarkers for SM, the hope is that patients will receive faster, more accurate diagnoses and more personalized treatment strategies based on their individual disease biology.