Beyond Serum Tryptase: Exploring Additional Testing Approaches for Indolent Systemic Mastocytosis

Tracy I. George, MD

Effective treatment of patients with indolent systemic mastocytosis begins with quickly and accurately identifying these patients through the proper use of multiple testing approaches, according to Tracy I. George, MD.

The most common mutation in indolent systemic mastocytosis is KIT D816V, which is present in over 80% of patients, making testing for this alteration standard practice.¹ Serum tryptase level is also an important indicator and is routinely tested.^2,3 However, these tests alone are sometimes not enough to diagnose a patient with the disease and nuances in the positivity cutoffs of clinically available tests can cloud results.

“If you are suspicious that a patient has indolent systemic mastocytosis, serum tryptase levels alone are not enough; I prefer the combination of a serum tryptase level screening and KIT D816V [testing] in the peripheral blood,” George said in an interview with OncLive®. “If you are highly suspicious, even if the serum tryptase level is normal, and [the patient’s] blood is negative for KIT D816V, I would suggest you still proceed to a bone marrow examination and do the appropriate genetic testing on that bone marrow sample.”

In November 2024, OncLive hosted a virtual workshop entitled Best Practices in Managing Indolent Systemic Mastocytosis (ISM) Disease Mechanisms, Diagnostic Challenges, and Multidisciplinary Treatment Strategies, where George was a part of a multidisciplinary panel of experts in the field moderated by Andrew Kuykendall, MD, of Moffitt Cancer Center. The panelists discussed the diverse clinical presentations of indolent systemic mastocytosis, testing criteria, essential diagnostic tests, and treatment options.

In the interview, George highlighted testing approaches to identify patients with indolent systemic mastocytosis that she shared during the event, the nuances of the various tests, and the effects that testing results have on a patient’s individual treatment plan. George is the president of the Innovation Business Unit, chief scientific officer at ARUP Laboratories, and a professor of pathology at the Spencer Fox Eccles School of Medicine at the University of Utah in Salt Lake City.

OncLive: What is the role of serum tryptase levels as a screening tool for indolent systemic mastocytosis?

George: Serum tryptase levels are very important as a screening tool for indolent systemic mastocytosis. Most patients with indolent systemic mastocytosis will have an elevated serum tryptase level greater than 20 ng/mL. However, there are a subset of patients whose serum tryptase levels are below that, or even within the normal values, and those are patients with very early involvement by indolent systemic mastocytosis. It’s a useful screening tool, but it won’t [properly identify] all of your patients.

The differential diagnosis of a patient who presents with an elevated serum tryptase level includes systemic mastocytosis, but it can also be seen in other conditions, [such as] renal failure or other myeloid neoplasms. It is [somewhat dependent] on how high the serum tryptase level is. For a serum tryptase level greater than 200 [ng/mL], a bone marrow examination [is needed]. [These patients will need] a bone marrow biopsy [and] KIT mutation testing, but most patients aren’t going to present in that [manner].

What other testing should be performed in patients with elevated serum tryptase levels?

[For most patients], a metabolic panel will clearly [rule out] renal failure. You’re going to ask patients about their signs and symptoms, and you will quickly ascertain whether they have symptoms that lead you to [consider] systemic mastocytosis. In order to make that diagnosis, you need to examine their bone marrow and as part of the workup for systemic mastocytosis it’s recommended that you test for KIT D816V using a highly sensitive method to approximately .01% variant allele frequency [VAF]. To do that, you need a very sensitive quantitative polymerase chain reaction [PCR] technique.

There are other research methods that are showing promise, but those are the clinical methods. For KIT mutation testing, it’s very important to look at not just the blood but also the bone marrow, because 15% of patients with indolent systemic mastocytosis will be negative in the blood but positive in the bone marrow. [Among] patients with bone marrow mastocytosis, almost 50% will be negative in the blood for KIT D816V but positive in the bone marrow. Depending on the subtype, you can get discordant results between blood and bone marrow [KIT testing results].

What other tests might you consider if a patient is suspected of having indolent systemic mastocytosis but has normal serum tryptase levels?

There are a subset of patients [with] indolent systemic mastocytosis [with] normal serum tryptase levels. For that reason, checking the blood for KIT D816V mutations, which are found in approximately 95% of patients with indolent systemic mastocytosis, is a great way to screen [these patients]. Alternatively, you may have a patient with hereditary alpha tryptasemia who has extra copies of the gene that encodes for alpha tryptase, and that’s why their serum tryptase levels are high. Since we can treat indolent systemic mastocytosis, we typically evaluate for that first [by] doing a bone marrow examination, and if that comes back negative for systemic mastocytosis, we will often reflex to doing testing for hereditary alpha tryptasemia.

What is the role of comprehensive and specific genetic testing in light of the prevalence of the KIT D816V mutation?

[This relates to the fact] that you have [often] discordant results between blood and bone marrow for KIT D816V depending on the subtype of systemic mastocytosis. For patients with indolent systemic mastocytosis, there is a false negative rate of approximately 15% in the blood using standard clinical testing. That’s 1 of the reasons we recommend that you also test bone marrow for KIT D816V using a highly sensitive method.

We also look for other myeloid gene mutations, especially in the blood and/or bone marrow of these patients, because some of these patients can have multimutated disease. These tend to be myeloid gene mutations, [such as] SRSF2, ASXL1, RUNX1, or TET2.

Patients who have more mutations generally don’t do as well as those who only have a KIT D816V mutation. Sometimes these [other] mutations can help indicate that the patient doesn’t have only indolent systemic mastocytosis, but they have an associated hematologic neoplasm, [such as] myelodysplastic syndrome or a myeloproliferative neoplasm. For that reason, we are often also getting next-generation sequencing [NGS] panels for many of these patients.

What testing methods could help identify any relevant alterations?

For KIT D816V we currently use ultra-sensitive methods in both blood and bone marrow, including digital droplet PCR and allele-specific oligonucleotide quantitative PCR. In general, we are looking at VAFs of .01% to .03%. There are more sensitive methods [used] in research labs, such as duplex sequencing or rolling circle amplification, but those are not generally in the clinical laboratory setting. There are interesting data that suggest these are very useful, especially in patients with indolent systemic mastocytosis.

If you’re only using NGS, you’re only going to get down to approximately 2% to 5% VAF. In my laboratory we used digital droplet PCR [to examine samples] from patients we knew had systemic mastocytosis and we found that over 80% had VAFs of less than 5%. So technically, if you use 5% as a cutoff for KIT D816V VAF for NGS, you’re going to miss most patients with systemic mastocytosis. That’s why understanding the testing methodology is very important.

How can testing results inform treatment planning?

It’s important to ensure that all these tests are done in a quality laboratory where they’re used to dealing with patients with systemic mastocytosis, because you use a bone marrow examination to determine if the patient has systemic mastocytosis and then you use [other] findings or morphologic assessment to figure out what subtype they have. Advanced systemic mastocytosis generally requires drugs that will reduce your mast cell burden, [such as] high-dosage TKIs.

[Comparatively, if] the patient has nonadvanced systemic mastocytosis, the majority of those are patients with indolent systemic mastocytosis. Those patients are treated with a selective TKI [such as] avapritinib [Ayvakit] at 25 mg per day. This is very different than the higher dose TKIs for advanced patients.

For more insights on diagnosing indolent systemic mastocytosis as well as to learn more about managing indolent systemic mastocytosis, read Best Practices in Managing Indolent Systemic Mastocytosis (ISM): Disease Mechanisms, Diagnostic Challenges, and Multidisciplinary Treatment Strategies here.

References

1. Ustun C, Arock M, Kluin-Nelemans HC, et al. Advanced systemic mastocytosis: from molecular and genetic progress to clinical practice. Haematologica. 2016;101(10):1133-1143. doi:10.3324/haematol.2016.146563
2. Greiner G, Sprinzl B, Górska A, et al. Hereditary α tryptasemia is a valid genetic biomarker for severe mediator-related symptoms in mastocytosis. Blood. 2021;137(2):238-247. doi:10.1182/blood.2020006157
3. Lyons JJ, Chovanec J, O’Connell MP, et al. Heritable risk for severe anaphylaxis associated with increased α-tryptase-encoding germline copy number at TPSAB1. J Allergy Clin Immunol. 2021;147(2):622-632. doi:10.1016/j.jaci.2020.06.035