Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) unleashes a multilayered inflammatory response in which epithelial injury, endothelial dysfunction, and microvascular thrombosis drive hypoxemic respiratory failure and multi-organ complications. Autopsy studies early in the pandemic established widespread pulmonary microangiopathy and intussusceptive angiogenesis as pathologic hallmarks of fatal COVID-19, underscoring the centrality of vascular–immune cross-talk to disease progression (Ackermann et al., 2020).

Among innate sentinels poised at the airway and perivascular interfaces, mast cells (MCs) have emerged as plausible amplifiers of COVID-19 pathology. Human and experimental data show MC activation in SARS-CoV-2 infection: lung tissue from severe/fatal cases contains increased tryptase+/chymase + MCs with ultrastructural evidence of degranulation, and circulating MC proteases (notably chymase and CPA3) are elevated and track with clinical severity (Tan et al., 2023; Krysko et al., 2022; Wismans et al., 2023). Tan et al. demonstrated in animal model that serum chymase, more than tryptase, associated with severe disease and lung pathology, whereas Krysko et al. reported that multiple MC proteases, including tryptase, rose with severity (Tan et al., 2023; Krysko et al.).

From a molecular perspective, tryptase—the dominant granule serine protease of human MCs—has several actions pertinent to COVID-19 pathobiology. Tryptase cleaves and activates protease-activated receptor-2 (PAR-2) on epithelial and endothelial cells, promoting pro-inflammatory cytokine release (e.g., IL-6, IL-8), increased permeability, and bronchoconstriction (Molino et al., 1997; Cicala and Cirino, 2002; Itoh et al., 2005). It can also contribute to matrix remodeling and fibrosis through proteolysis of extracellular components and PAR-2–dependent signaling (Cicala and Cirino, 2002; Itoh et al., 2005). With respect to coagulation, tryptase–heparin complexes can modulate clot architecture and fibrinolysis, and experimental systems indicate effects on thrombus biology that dovetail with the immunothrombotic features of COVID-19 (Prieto-García et al., 2012; Samoszuk et al., 2003).

Yet the clinical performance of serum tryptase as a biomarker in COVID-19 has been inconsistent across studies (Krysko et al., 2022; Wismans et al., 2023). Two features likely contribute. First, sampling kinetics: acute tryptase peaks are short-lived (biological half-life ∼2 h), so blood drawn outside a narrow window may miss elevations (Valent et al., 2023). Second, compartmentalization: MC activation may be tissue-restricted (airway/lung), with minimal “spill-over” to blood—consistent with stronger signals for chymase/CPA3 than for tryptase in several cohorts (Tan et al., 2023; Krysko et al., 2022; Wismans et al., 2023).

Tryptase is a mast cell mediator well known for its role in allergic reactions, but its role in viral infections remains unclear. To place this in context, we compared tryptase with biomarkers already established in COVID-19 prognosis. A large systematic review and meta-analysis showed that inflammatory markers such as CRP, PCT, and LDH are consistently elevated in patients with severe COVID-19 and are associated with higher mortality (Hariyanto et al., 2020). In addition, Henry and colleagues, in an early pooled analysis, reported that elevated LDH and CRP were among the strongest laboratory predictors of severe and fatal disease (Henry et al., 2020). More recently, a meta-analysis focused specifically on procalcitonin confirmed its strong prognostic value, with pooled sensitivity and specificity values indicating meaningful predictive accuracy for both severity and mortality (Kumar et al., 2022). Against this backdrop, our study sought to evaluate whether tryptase, as a mast cell–derived biomarker, might provide complementary prognostic information to these established markers. In this study, we asked whether serum tryptase measured upon hospital admission could serve as a prognostic biomarker in COVID-19, specifically whether it could identify patients at higher risk of requiring oxygen therapy or in-hospital death.

A total of 82 patients hospitalized with confirmed SARS-CoV-2 infection were enrolled in the study. Of these, 20 patients (24.4%) were classified as oxygen-independent, while 61 patients (74.4%) required oxygen therapy during hospitalization and were categorized as oxygen-dependent. Subgroup analyses were performed by stratifying patients according to oxygen dependency and survival status, which reflect clinically meaningful severity categories. Additional subgrouping into WHO-defined severity classes was not performed due to limited sample size.

Statistical analysis revealed no significant differences between the two groups regarding age, sex, body mass index (BMI), or hospitalization duration. Detailed baseline characteristics are presented in Table 1.

The two groups showed statistically significant differences in CRP, PCT, and LDH levels, with higher values observed in the oxygen-dependent group. No significant differences were found between groups regarding tryptase and lymphocyte counts. Detailed laboratory parameters are presented in Table 2.

Patients who died were significantly older compared to survivors (mean 77.87 vs. 64.64 years, p = 0.0057). No statistically significant differences were observed between the groups regarding serum tryptase or CRP concentrations. However, deceased patients demonstrated significantly higher PCT levels (mean 3.08 vs. 1.23 ng/mL, p = 0.0016) and elevated LDH activity (mean 563.60 vs. 347.07 U/L, p = 0.0360). Detailed comparisons of demographic, biochemical, and hematological parameters are provided in Table 3.

The ROC analysis revealed that C-reactive protein (CRP), procalcitonin (PCT), and lactate dehydrogenase (LDH) were statistically significant predictors of the need for oxygen therapy. CRP demonstrated the strongest discriminative ability, with an AUC of 0.758 (p < 0.0001) and an optimal cut-off point of 20.70 mg/L. PCT and LDH also showed significant predictive value, with AUCs of 0.717 (p = 0.0059) and 0.727 (p = 0.0058), and cut-off points of 0.04 ng/mL and 193 U/L, respectively.

Patients with CRP levels above 20.70 mg/L, PCT levels above 0.04 ng/mL, or LDH levels above 193 U/L were more likely to require oxygen supplementation during hospitalization.

Although the calculated sensitivities were moderate to high, the specificities were relatively low.

Moreover, pairwise comparisons of ROC curves demonstrated that CRP was a statistically superior classifier compared to both PCT and LDH (p < 0.05), indicating its higher accuracy in distinguishing between patients requiring and not requiring oxygen therapy.

A detailed summary of the ROC parameters is presented in Table 4.

The ROC analysis demonstrated that age, procalcitonin (PCT), and lactate dehydrogenase (LDH) were statistically significant predictors of in-hospital mortality. Age showed moderate predictive ability (AUC = 0.736), while PCT showed the highest discriminative power (AUC = 0.814). LDH also demonstrated acceptable prognostic value with an AUC of 0.714.

The optimal cut-off points for age, PCT, and LDH were 84 years, 0.43 ng/mL, and 814 U/L, respectively. Although the sensitivities were modest, all three parameters achieved high specificities exceeding 90%.

In contrast, tryptase, CRP, and lymphocyte count did not show significant discriminative performance (AUC <0.7; p > 0.05).

Comparative analysis revealed no statistically significant differences between the AUC values of age, PCT, and LDH (p > 0.05), indicating similar predictive accuracies.

Detailed ROC metrics are summarized in Table 5.

The odds ratio analysis identified age over 65 years (OR = 5.60, p = 0.0155), BMI below 18.5 (OR = 10.00, p = 0.0340), and PCT above 0.1 ng/mL (OR = 15.00, p = 0.0068) as statistically significant predictors of in-hospital mortality among COVID-19 patients. Other factors, including gender, hospitalization duration, tryptase levels, CRP, LDH, and lymphocyte counts, did not show statistically significant associations with mortality. The results of the univariate odds ratio analysis for predictors of in-hospital mortality are presented in Table 6.

The median hospital stay did not differ between groups, and no significant associations were found between tryptase levels and either length of hospitalization or the occurrence of complications.

The results of our study confirm the prognostic relevance of classical inflammatory markers in COVID-19 and underscore the limited utility of serum tryptase measured at admission. Elevated CRP, PCT, and LDH were strongly associated with severity and mortality in our cohort, consistent with multiple meta-analyses and large observational studies (Hariyanto et al., 2020; Henry et al., 2020; Kumar et al., 2022; Stringer et al., 2021). For instance, Henry et al. demonstrated early in the pandemic that high CRP and LDH were robust predictors of mortality (Henry et al., 2020), and Kumar et al. confirmed that procalcitonin retained independent prognostic value (Kumar et al., 2022). Our results reinforce the clinical utility of these inexpensive and widely available biomarkers for early risk stratification.

By contrast, serum tryptase did not discriminate outcomes in our study. Although tryptase is a biologically plausible marker of mast cell activation and may play a mechanistic role in COVID-19–related inflammation, its clinical utility as a circulating biomarker appears limited. This may reflect the short biological half-life of mature tryptase (approximately 2 hours) and the fact that mast cell activation in COVID-19 is largely tissue-restricted, with minimal release of tryptase into the circulation. Consequently, transient local increases in tryptase may not be captured in single-timepoint serum measurements obtained at hospital admission. It is therefore possible that tryptase could serve as a valuable prognostic marker if measured at multiple time points, which might help to identify the optimal sampling window for detecting transient increases associated with disease progression.

This echoes findings from Tan et al., who showed that chymase, not tryptase, was the mast cell protease most consistently elevated in severe COVID-19 (Tan et al., 2023). Krysko et al. reported increases in tryptase, chymase, and CPA3 correlating with severity, though the effect size for tryptase was smaller. In contrast to the studies by Tan et al. and Krysko et al. which primarily investigated patients with severe or fatal COVID-19—and, in part, post-mortem tissue samples—our cohort included a broader clinical spectrum ranging from mild to severe cases, with serum samples collected exclusively at hospital admission. In the study by Tan et al., key observations were derived mainly from animal models and histopathological analyses demonstrating extensive mast cell activation within the lungs during severe infection. Similarly, Krysko et al. reported increased numbers of alveolar mast cells and elevated levels of mast cell proteases (chymase, CPA3, and tryptase) in patients with severe disease or post-mortem specimens. These differences in disease severity and sampling timing may partly explain why serum tryptase in our study did not show prognostic significance despite its mechanistic relevance in severe COVID-19.

Several pathophysiological considerations support this conclusion. Tryptase has a short half-life (∼2 h), so single-admission samples are unlikely to capture transient surges (Valent et al., 2023). Moreover, mast cell activation in COVID-19 appears to be predominantly tissue-restricted. Autopsy studies demonstrate increased numbers of tryptase+ and chymase + mast cells in COVID-19 lungs, with evidence of degranulation particularly in areas of thrombosis, angiogenesis, and fibrosis (Ackermann et al., 2020; Wismans et al., 2023; Meneses-Preza et al., 2024). This aligns with the vascular pathology described by Ackermann et al., who identified endothelialitis, widespread thrombosis, and intussusceptive angiogenesis as defining features of fatal COVID-19 (Ackermann et al., 2020). Thus, mast cell activity may be highly relevant at the local tissue level but may not be reflected adequately in peripheral blood tryptase.

The discrepancy between tissue pathology and circulating tryptase also highlights the potential for biased mediator release. Mast cells can secrete specific subsets of mediators—including cytokines, chemokines, and lipid mediators—without concomitant release of tryptase, a phenomenon described as “piecemeal degranulation” (Molino et al., 1997; Cicala and Cirino, 2002; Itoh et al., 2005). Indeed, experimental data indicate that SARS-CoV-2 proteins alone do not directly trigger mast cell degranulation, suggesting that activation is secondary to the cytokine milieu and tissue injury (Krysko et al., 2022). This could explain why COVID-19 exhibits clinical and histological features of mast cell activation without corresponding elevations in serum tryptase.

From a mechanistic standpoint, tryptase retains clear pathogenetic relevance. It activates protease-activated receptor-2 (PAR-2) on airway epithelium and endothelium, inducing IL-6 and IL-8 release, barrier dysfunction, and bronchoconstriction (Molino et al., 1997; Cicala and Cirino, 2002; Itoh et al., 2005). It also remodels the extracellular matrix and, together with heparin, modifies fibrin architecture, thereby promoting fibrosis and thrombosis (Prieto-García et al., 2012; Samoszuk et al., 2003). These effects dovetail with the immunothrombotic features of severe COVID-19 and provide a plausible link between mast cell activation and the vascular–fibrotic pathology described in post-mortem lungs (Ackermann et al., 2020; Wismans et al., 2023; Meneses-Preza et al., 2024).

Therapeutically, these insights suggest that mast cell–directed interventions may hold promise. Preclinical work demonstrated that mast cell stabilizers attenuate SARS-CoV-2–induced epithelial inflammation and lung injury (Tan et al., 2023). Observational data have also raised interest in histamine receptor antagonists, including famotidine, which in some studies was associated with improved outcomes, though results remain inconclusive (Freedberg et al., 2020; Janowitz et al., 2022). To translate these insights into clinical practice, better biomarkers are needed. Panels incorporating chymase, CPA3, and prostaglandin D₂ metabolites, in addition to tryptase, may better capture mast cell activation and identify biomarker-enriched subgroups for targeted therapies.

In summary, our findings support the central role of mast cells in COVID-19 pathogenesis but highlight that serum tryptase alone is not a reliable prognostic marker. Classical inflammatory parameters (CRP, PCT, LDH) remain more informative for clinical risk stratification, while future research should focus on composite mast cell signatures and carefully timed sampling to guide personalized therapeutic approaches.