Diminished reliability of tryptase as risk indicator of mastocytosis in older overweight subjects

Background: Currently, measurement of serum tryptase level is the most commonly used test to estimate the need for bone marrow biopsy in patients suspected to have indolent systemic mastocytosis (ISM). Yet tryptase levels do not solely reflect the mast cell load and can be elevated by overweight, older age, and impaired renal function. The influence of these factors on urinary methylhistamine (MH) and methylimidazole acetic acid (MIMA) is still unknown. Objective: We investigated the impact of age, body mass index (BMI), and kidney function on the diagnostic accuracy of tryptase, MH, and MIMA to select the most optimal test indicating the necessity of a bone marrow biopsy in ISM-suspected patients. Methods: Retrospective data analysis of all adults in whom bone marrow investigations were performed because of high clinical suspicion and/or elevated tryptase, MH, or MIMA. Results: 194 subjects were included. ISM was present in 112 and absent in 82 subjects (non-ISM). Tryptase was elevated by age and body weight in non-ISM subjects and by BMI in ISM subjects; however, these factors did not influence MH or MIMA. In the total study population, the diagnostic accuracy of tryptase, MH, and MIMA were comparable (area under the curve 0.80, 0.80, and 0.83). In subjects > 50 years with a BMI > 25 kg/m(2), the diagnostic accuracy of MIMA was higher compared with that of tryptase (area under the curve 0.93 vs 0.74; P = .011). Conclusion: In ISM-suspected patients > 50 years with a BMI of > 25 kg/m(2), MIMA has a greater value compared with tryptase in estimating the need for bone marrow biopsy