A critical view on autoantibodies in lupus nephritis: Concrete knowledge based on evidence.

Deposition of autoantibodies in glomeruli is a key factor in the development of lupus nephritis (LN). For a long time, anti-dsDNA and anti-C1q antibodies were thought to be the main cause of the kidney damage. However, recent studies have shown that the list of autoantibidies that have renal tropism and deposit in the kidney in LN is increasing and the link between anti-dsDNA and renal pathology is weak due to potential confounders. Aspecific bindings of dsDNA with cationic antibodies and of anti-dsDNA with several renal antigens such as actinin, laminin, entactin, and annexinA2 raised doubts about the specific target of these antibodies in the kidney. Moreover, the isotype of anti-dsDNA in SLE and LN has never received adequate interest until the recent observation that IgG2 are preponderant over IgG1, IgG3 and IgG4. Based on the above background, recent studies investigated the involvement of anti-dsDNA IgG2 and of other antibodies in LN. It was concluded that circulating anti-dsDNA IgG2 levels do not distinguish between LN versus non-renal SLE, and, in patients with LN, their levels do not change over time. Circulating levels of other antibodies such as anti-ENO1 and anti-H2 IgG2 were, instead, higher in LN vs non-renal SLE at the time of diagnosis and decreased following therapies. Finally, new classes of renal antibodies that potentially modify the anti-inflammatory response in the kidney are emerging as new co-actors in the pathogenetic scenario. They have been defined as 'second wave antibodies' for the link with detoxifying mechanisms limiting the oxidative stress in glomeruli that are classically stimulated in a second phase of inflammation. These findings have important clinical implications that may modify the laboratory approach to LN. Serum levels of anti-ENO1 and anti-H2 IgG2 should be measured in the follow up of patients for designing the length of therapies and identify those patients who respond to treatments. Anti-SOD2 could help to monitor and potentiate the anti-inflammatory response in the kidney.

Sustainability in Internal Medicine: A Year-Long Ward-Wide Observational Study.

Population aging and multimorbidity challenge health system sustainability, but the role of assistance-related variables rather than individual pathophysiological factors in determining patient outcomes is unclear. To identify assistance-related determinants of sustainable hospital healthcare, all patients hospitalised in an Internal Medicine Unit (n = 1073) were enrolled in a prospective year-long observational study and split 2:1 into a training (n = 726) and a validation subset (n = 347). Demographics, comorbidities, provenance setting, estimates of complexity (cumulative illness rating scale, CIRS: total, comorbidity, CIRS-CI, and severity, CIRS-SI subscores) and intensity of care (nine equivalents of manpower score, NEMS) were analysed at individual and Unit levels along with variations in healthcare personnel as determinants of in-hospital mortality, length of stay and nosocomial infections. Advanced age, higher CIRS-SI, end-stage cancer, and the absence of immune-mediated diseases were correlated with higher mortality. Admission from nursing homes or intensive care units, dependency on activity of daily living, community- or hospital-acquired infections, oxygen support and the number of exits from the Unit along with patient/physician ratios were associated with prolonged hospitalisations. Upper gastrointestinal tract disorders, advanced age and higher CIRS-SI were associated with nosocomial infections. In addition to demographic variables and multimorbidity, physician number and assistance context affect hospitalisation outcomes and healthcare sustainability.

Histone-Specific CD4+ T Cell Plasticity in Active and Quiescent Systemic Lupus Erythematosus.

OBJECTIVE: The aim of this study was to assess whether circulating histone-specific T cells represent tools for precision medicine in systemic lupus erythematosus (SLE). METHODS: Seroprevalence of autoantibodies and HLA-DR beta (DRB) 1 profile were assessed among 185 patients with SLE and combined with bioinformatics and literature evidence to identify HLA-peptide autoepitope couples for ex vivo detection of antigen-specific T cells through flow cytometry. T cell differentiation and polarization was investigated in patients with SLE, patients with Takayasu arteritis, and healthy controls carrying HLA-DRB1*03:01 and/or HLA-DRB1*11:01. SLE Disease Activity Index 2000 and Lupus Low Disease Activity State were used to estimate disease activity and remission. RESULTS: Histone-specific CD4+ T cells were selectively detected in patients with SLE. Among patients with a history of anti-DNA antibodies, 77% had detectable histone-specific T cells, whereas 50% had lymphocytes releasing cytokines or upregulating activation markers after in vitro challenge with histone peptide antigens. Histone-specific regulatory and effector T helper (Th) 1-, Th2-, and atypical Th1/Th17 (Th1*)-polarized cells were significantly more abundant in patients with SLE with quiescent disease. In contrast, total Th1-, Th2-, and Th1*-polarized and regulatory T cells were similarly represented between patients and controls or patients with SLE with active versus quiescent disease. Histone-specific effector memory T cells accumulated in the blood of patients with quiescent SLE, whereas total effector memory T cell counts did not change. Immunosuppressants were associated with expanded CD4+ histone-specific naive T (TN) and terminally differentiated T cells. CONCLUSION: Histone-specific T cells are selectively detected in patients with SLE, and their concentration in the blood varies with disease activity, suggesting that they represent innovative tools for patient stratification and therapy.

Choroid plexus and perivascular space enlargement in neuropsychiatric systemic lupus erythematosus.

Choroid plexus (CP) enlargement is proposed as a marker of neuroinflammation in immune-mediated conditions. CP involvement has also been hypothesized in the immunopathology of systemic lupus erythematosus (SLE). We investigated whether CP enlargement occurs in SLE patients and its association with neuropsychiatric involvement. Additionally, we explored abnormalities along the glymphatic system in SLE patients through enlarged perivascular space (PVS) quantification. Clinical assessment and 3 Tesla brain dual-echo and T1-weighted MRI scans were obtained from 32 SLE patients and 32 sex and age-matched healthy controls (HC). CPs were manually segmented on 3D T1-weighted sequence and enlarged PVS (ePVS) were assessed through Potter's score. Compared to HC, SLE patients showed higher normalized CP volume (nCPV) (p = 0.023), with higher CP enlargement in neuropsychiatric SLE (NPSLE) (n = 12) vs. non-NPSLE (p = 0.027) patients. SLE patients with antiphospholipid antibodies (APA) positivity (n = 18) had higher nCPV compared to HC (p = 0.012), while APA negative ones did not. SLE patients also had higher Potter's score than HC (p < 0.001), with a tendency towards a higher number of basal ganglia ePVS in NPSLE vs. non-NPSLE patients. Using a random forest analysis, nCPV emerged as a significant predictor of NPSLE, together with T2-hyperintense white matter (WM) lesion volume (LV) and APA positivity (out-of-bag AUC 0.81). Our findings support the hypothesis of a role exerted by the CP in SLE physiopathology, especially in patients with neuropsychiatric involvement. The higher prevalence of ePVS in SLE patients, compared to HC, suggests the presence of glymphatic system impairment in this population.

Anifrolumab for Moderate and Severe Muco-Cutaneous Lupus Erythematosus: A Monocentric Experience and Review of the Current Literature.

Refractory cutaneous manifestations constitute a significant unmet need in patients with cutaneous lupus (CLE), even in the setting of systemic lupus erythematosus (SLE) with otherwise good control of inflammatory manifestations. Anifrolumab, an anti-interferon I receptor monoclonal antibody has recently been approved for serologically positive SLE with or without CLE, but real-life efficacy and safety data are currently limited. In addition, relatively limited evidence exists about the spectrum of cutaneous manifestations potentially benefitting from anifrolumab treatment and about the optimal clinimetrics to monitor treatment efficacy. While summarising current evidence on the topic in the literature, we report on four patients with SLE and refractory CLE who were successfully treated with anifrolumab. We also describe the potential usefulness and complementarity of the cutaneous lupus activity investigator's global assessment (CLA-IGA) in assessing cutaneous activity in patients treated with anifrolumab.

Is SARS-CoV-2-induced disease a decisive factor influencing testosterone in males? Findings from a case-control ex post facto study.

BACKGROUND: Whether the observed lower total testosterone (tT) levels in male patients with COVID-19 are caused by a direct impact of SARS-CoV-2 infection or are collateral phenomena shared by other systemic inflammatory conditions has not yet been clarified. OBJECTIVES: To investigate the independent role of COVID-19 in reducing circulating tT levels in men. MATERIALS AND METHODS: We compared demographic, clinical, and hormonal values of patients with laboratory confirmed COVID-19 admitted during the first wave of the pandemic with a cohort of consecutive male patients admitted to the intensive care unit (ICU) of the same academic center because of severe acute respiratory distress syndrome (ARDS) but without SARS-CoV-2 infection and no previous history of COVID-19. Linear regression model tested the independent impact of COVID-19 on circulating tT levels. Logistic regression model was used to test predictors of death in the entire cohort. RESULTS: Of 286 patients with COVID-19, 70 men had been admitted to the ICU ( = cases) and were compared to 79 patients equally admitted to ICU because of severe ARDS but negative for SARS-CoV-2 infection and without previous history of COVID-19 ( = controls). Controls were further grouped into noninfective (n = 49) and infective-ARDS (n = 30) patients. At baseline, controls were older (p = 0.01) and had more comorbidities (p < 0.0001). Overall, cases admitted to ICU had significantly lower circulating tT levels compared to controls (0.9 nmol/L vs. 2.1 nmol/L; vs. 1.2 nmol/L; p = 0.03). At linear regression, being negative for COVID-19 was associated with higher tT levels (Coeff: 2.13; 95% confidence interval - CI 0.71-3.56; p = 0.004) after adjusting for age, BMI, comorbidities and IL-6 levels. Only age and IL-6 levels emerged to be associated with higher risk of death regardless of COVID-19 status. CONCLUSIONS: This case-control ex post facto study showed lower tT levels in men with COVID-19 compared to those without COVID-19 despite both groups have been equally admitted to ICU for severe ARDS, thus suggesting a possible direct impact of SARS-CoV-2 infection toward circulating tT levels and a consequent more severe clinical outcome.

Efficacy and Safety of Anti-SARS-CoV-2 Antiviral Agents and Monoclonal Antibodies in Patients with SLE: A Case-Control Study.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-related disease (COVID-19) has spread pandemically with high rates of morbidity and mortality. COVID-19 has also posed unprecedented challenges in terms of rapid development of pharmacological countermeasures to prevent or contrast SARS-CoV-2 pathogenicity. Anti-SARS-CoV-2 antiviral agents and monoclonal antibodies have been specifically designed to attenuate COVID-19 morbidity and prevent mortality in vulnerable subjects, such as patients with immune-mediated diseases, but evidence for the safe and effective use of these drugs in this latter population group is scarce. Therefore, we designed a retrospective, multicentre, observational, case-control study to analyse the impact of these treatments in COVID-19 patients with systemic lupus erythematosus (SLE), a paradigmatic, multi-organ autoimmune disease. We identified 21 subjects treated with antivirals and/or monoclonal antibodies who were matched with 42 untreated patients by age, sex, SLE extension and duration. Treated patients had higher baseline SLE disease activity index 2000 scores [SLEDAI-2K median (interquartile range) = 4 (1-5) vs. 0 (0-2); p = 0.009], higher prednisone doses [5 (0-10) mg vs. 0 (0-3) mg; p = 0.002], and more severe COVID-19 symptoms by a five-point World Health Organisation-endorsed analogue scale [1 (0-1) vs. 0 (0-1); p < 0.010] compared to untreated patients. There was no difference between groups in terms of COVID-19 outcomes and sequelae, nor in terms of post-COVID-19 SLE exacerbations. Three subjects reported mild adverse events (two with monoclonal antibodies, one with nirmatrelvir/ritonavir). These data suggest that anti-SARS-CoV-2 antivirals and monoclonal antibodies might be safely and effectively used in patients with SLE, especially with active disease and more severe COVID-19 symptoms at presentation.

Role of systemic immunosuppression on subglottic stenosis in granulomatosis with polyangiitis: Analysis of a single-centre cohort.

PURPOSE: Subglottic stenosis (SGS) is a potentially life-threatening manifestation of granulomatosis with polyangiitis (GPA). Endoscopic dilation is effective, but relapses are frequent and the benefit of systemic immunosuppression in this setting is still controversial. We aimed to investigate the role of immunosuppressive treatment on SGS relapse risk. METHODS: This is a retrospective observational study based on review of medical charts among our cohort of patients with GPA. RESULTS: Twenty-one patients with SGS-GPA were identified, with a prevalence of 20% among our entire GPA cohort (n = 105). Compared to patients without SGS, patients with SGS-GPA had an earlier disease onset (mean age 30.2 vs. 47.3 years, p<0.001), and lower BVAS (mean 10.5 vs 13.5; p = 0.018). Five patients didn't receive systemic immunosuppression for SGS and they all (100%) relapsed after the first procedure, while among medical treatment group relapse rate was 44% (p = 0.045). When single treatment regimens are considered, rituximab (RTX) and cyclophosphamide (CYC) yielded a protective role towards the need of subsequent dilation procedure after the first if compared with absence of medical treatment. Patients with SGS and generalized disease, who initially received either a RTX- or a CYC-based induction treatment, and higher cumulative doses of glucocorticoids, showed a delayed median time to SGS relapse (36 vs. 12 months, p = 0.024). CONCLUSIONS: Subglottic stenosis is highly prevalent in patients with GPA and may define a milder systemic disease subset occurring more frequently in younger patients. Systemic immunosuppression provides benefit in preventing recurrence of SGS in GPA patients and regimens based on cyclophosphamide or rituximab might have a non-redundant role in this setting.

Two-year multidisciplinary follow-up of COVID-19 patients requiring invasive and noninvasive respiratory support.

BACKGROUND: COVID-19 patients frequently develop respiratory failure requiring mechanical ventilation. Data on long-term survival of patients who had severe COVID-19 are insufficient. We assessed and compared two-year survival, CT imaging, quality of life, and functional recovery of COVID-19 ARDS patients requiring respiratory support with invasive (IMV) versus noninvasive ventilation (NIV). METHODS: Patients with COVID-19 pneumonia admitted up to May 28th, 2020, who required IMV or NIV, and survived to hospital discharge were enrolled. Patients were contacted two years after discharge to assess vital status, functional, psychological, and cognitive outcomes using validated scales. Patients with persistent respiratory symptoms or high burden of residual lung damage at previous CT scan received a two-year chest CT scan. RESULTS: Out of 61 IMV survivors, 98% were alive at two-year follow-up, and 52 completed the questionnaire. Out of 82 survivors receiving NIV only, 94% were alive at two years, and 47 completed the questionnaire. We found no major differences between invasively and noninvasively ventilated patients, with overall acceptable functional recovery. Among the 99 patients completing the questionnaire, 23 have more than moderate exertional dyspnea. Chest CT scans showed that 4 patients (all received IMV) had fibrotic-like changes. CONCLUSIONS: Patients who received mechanical ventilation due to COVID-19 and were discharged from hospital had a 96% survival rate at the two-year follow-up. There was no difference in overall recovery and quality of life between patients who did and did not require IMV, although respiratory morbidity remains high.

Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): Focus on the Pathophysiological and Diagnostic Role of Viruses.

Drug reaction with eosinophilia and systemic symptoms (DRESS) is a heterogeneous, multiorgan and potentially life-threatening drug-hypersensitivity reaction (DHR) that occurs several days or weeks after drug initiation or discontinuation. DHRs constitute an emerging issue for public health, due to population aging, growing multi-organ morbidity, and subsequent enhanced drug prescriptions. DRESS has more consistently been associated with anticonvulsants, allopurinol and antibiotics, such as sulphonamides and vancomycin, although new drugs are increasingly reported as culprit agents. Reactivation of latent infectious agents such as viruses (especially Herpesviridae) plays a key role in prompting and sustaining aberrant T-cell and eosinophil responses to drugs and pathogens, ultimately causing organ damage. However, the boundaries of the impact of viral agents in the pathophysiology of DRESS are still ill-defined. Along with growing awareness of the multifaceted aspects of immune perturbation caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during the ongoing SARS-CoV-2-related disease (COVID-19) pandemic, novel interest has been sparked towards DRESS and the potential interactions among antiviral and anti-drug inflammatory responses. In this review, we summarised the most recent evidence on pathophysiological mechanisms, diagnostic approaches, and clinical management of DRESS with the aim of increasing awareness on this syndrome and possibly suggesting clues for future research in this field.

Testosterone in males with COVID-19: a 12-month cohort study.

BACKGROUND: Male patients with COVID-19 have been found with reduced serum total testosterone (tT) levels and with more severe clinical outcomes. OBJECTIVES: To assess total testosterone (tT) levels and the probability of recovering eugonadal tT levels during a minimum 12-month timespan in a cohort of men who have been followed over time after the recovery from laboratory-confirmed COVID-19. MATERIALS AND METHODS: Demographic, clinical and hormonal values were collected for the overall cohort. Hypogonadism was defined as tT ≤9.2 nmol/l. The Charlson Comorbidity Index was used to score health-significant comorbidities. Descriptive statistics was used to compare hormonal levels at baseline versus 7-month (FU1) versus 12-month (FU2) follow-up, respectively. Multivariate cox proportional hazards regression model was used to identify the potential predictors of eugonadism recovery over time among patients with hypogonadism at the time of infection. RESULTS: Of the original cohort of 286 patients, follow-up data were available for 121 (42.3%) at FU1 and 63 (22%) patients at FU2, respectively. Higher median interquartile range (IQR) tT levels were detected at FU2 (13.8 (12.3-15.3) nmol/L) versus FU1 (10.2 [9.3-10.9] nmol/L) and versus baseline (3.6 [3.02-4.02] nmol/L) (all p < 0.0001), whilst both LH and E2 levels significantly decreased over the same time frame (all p ≤ 0.01). Circulating IL-6 levels further decreased at FU2 compared to FU1 levels (19.3 vs. 72.8 pg/ml) (p = 0.02). At multivariable cox regression analyses, baseline tT level (HR 1.19; p = 0.03 [1.02-1.4]) was independently associated with the probability of tT level normalization over time, after adjusting for potential confounders. CONCLUSIONS: Circulating tT levels keep increasing over time in men after COVID-19. Still, almost 30% of men who recovered from COVID-19 had low circulating T levels suggestive for a condition of hypogonadism at a minimum 12-month follow-up.

Combined plasma levels of IL-10 and testosterone, but not soluble HLA-G5, predict the risk of death in COVID-19 patients.

BACKGROUND: The identification of biomarkers correlated with coronavirus disease 2019 (COVID-19) outcomes is a relevant need for clinical management. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is characterized by elevated interleukin (IL)-6, IL-10, HLA-G, and impaired testosterone production. OBJECTIVES: We aimed at defining the combined impact of sex hormones, interleukin-10, and HLA-G on COVID-19 pathophysiology and their relationship in male patients. MATERIALS AND METHODS: We measured by chemiluminescence immunoassay, electrochemiluminescent assays, and enzyme-linked immunosorbent assay circulating total testosterone, 17ß-estradiol (E2 ), IL-10, and -HLAG5 as well as SARS-CoV-2 S1/S2 Immunoglobulin G from 292 healthy controls and 111 COVID-19 patients with different disease severity at hospital admission, and in 53 COVID-19 patients at 7-month follow-up. RESULTS AND DISCUSSION: We found significantly higher levels of IL-10, HLA-G, and E2 in COVID-19 patients compared to healthy controls and an inverse correlation between IL-10 and testosterone, with IL-10, progressively increasing and testosterone progressively decreasing with disease severity. This correlation was lost at the 7-month follow-up. The risk of death in COVID-19 patients with low testosterone increased in the presence of high IL-10. A negative correlation between SARS-CoV-2 Immunoglobulin G and HLA-G or IL-10 at hospitalization was observed. At the 7-month follow-up, IL-10 and testosterone normalized, and  HLA-G decreased. CONCLUSION: Our findings indicate that combined evaluation of IL-10 and testosterone predicts the risk of death in men with COVID-19 and support the hypothesis that IL-10 fails to suppress excessive inflammation by promoting viral spreading.

Interactions between Severe Allergy and Anxiety in Anti-SARS-CoV-2 Vaccinees.

Severe drug allergy affects patient hesitancy to new treatments, posing unprecedented challenges to anti-SARS-CoV-2 vaccination campaigns. We aimed to analyze the psychological profile of vaccinees with a history of severe allergy in comparison to subjects with a milder allergy history. Patients attending a dedicated vaccination setting were administered an anonymized questionnaire including clinical data and the State-Trait Anxiety Inventory (STAI) scale (score range 20−80). Patients were also asked whether being in a protected setting affected their attitude toward vaccination. Data are expressed as median (interquartile range). We enrolled 116 patients (78% women), of whom 79% had a history of drug anaphylaxis. The median state anxiety score was 36.5 (30−47.2), while the trait anxiety score was 37 (32−48). State anxiety was higher in those with severe than mild allergy [39 (32−50) vs. 30 (25−37); p < 0.001], with the highest score found in a patient with previous drug anaphylaxis (42.5 [32−51.7]). More than 50% of patients reported that being in a protected setting had lowered their anxiety. Severe allergy is associated with a higher burden of situational anxiety in the setting of vaccination without affecting patient constitutional (trait) levels of anxiety. Vaccination in dedicated facilities might overcome issues related to hesitancy and improve patients' quality of life.

Cardiac Safety of mRNA-Based Vaccines in Patients with Systemic Lupus Erythematosus and Lupus-like Disorders with a History of Myocarditis.

Anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines may trigger immune-mediated adverse events, including myocarditis. Evidence of vaccine safety in patients with rheumatic disorders and underlying autoimmune myocarditis is scarce. To address this issue, we studied 13 patients with systemic lupus erythematosus (SLE) and allied conditions with a history of myocarditis and receiving mRNA-based vaccines. Data about general and cardiac laboratory tests, treatment, and disease status were collected during routine consultations before and after the primary vaccination course and after each vaccine dose administration, while myocarditis symptoms were closely monitored. A significant increase in troponin levels from baseline was found after 13 (6-20) days from the first (p = 0.046) and 17 (4-29) days after the second dose (p = 0.013). Troponin levels progressively decreased within 3 (1-6) months in the absence of typical symptoms or signs of myocarditis. A significant increase in the constitutional domain of the British Isles Lupus Assessment Group (BILAG) index (p = 0.046) was observed in SLE patients. However, no patient needed any treatment change. mRNA-based anti-SARS-CoV-2 vaccines can apparently be safely administered to patients with SLE and lupus-like disorders with previous myocarditis despite potential subclinical and transient rises in cardiac damage markers.

Chromogranin A plasma levels predict mortality in COVID-19.

BACKGROUND: Chromogranin A (CgA) and its fragment vasostatin I (VS-I) are secreted in the blood by endocrine/neuroendocrine cells and regulate stress responses. Their involvement in Coronavirus 2019 disease (COVID-19) has not been investigated. METHODS: CgA and VS-I plasma concentrations were measured at hospital admission from March to May 2020 in 190 patients. 40 age- and sex-matched healthy volunteers served as controls. CgA and VS-I levels relationship with demographics, comorbidities and disease severity was assessed through Mann Whitney U test or Spearman correlation test. Cox regression analysis and Kaplan Meier survival curves were performed to investigate the impact of the CgA and VS-I levels on in-hospital mortality. RESULTS: Median CgA and VS-I levels were higher in patients than in healthy controls (CgA: 0.558 nM [interquartile range, IQR 0.358-1.046] vs 0.368 nM [IQR 0.288-0.490] respectively, p = 0.0017; VS-I: 0.357 nM [IQR 0.196-0.465] vs 0.144 nM [0.144-0.156] respectively, p<0.0001). Concentration of CgA, but not of VS-I, significantly increased in patients who died (n = 47) than in survivors (n = 143) (median 0.948 nM [IQR 0.514-1.754] vs 0.507 nM [IQR 0.343-0.785], p = 0.00026). Levels of CgA were independent predictors of in-hospital mortality (hazard ratio 1.28 [95% confidence interval 1.077-1.522], p = 0.005) when adjusted for age, number of comorbidities, respiratory insufficiency degree, C-reactive protein levels and time from symptom onset to sampling. Kaplan Meier curves revealed a significantly increased mortality rate in patients with CgA levels above 0.558 nM (median value, log rank test, p = 0.001). CONCLUSION: Plasma CgA levels increase in COVID-19 patients and represent an early independent predictor of mortality.

Basal Serum Diamine Oxidase Levels as a Biomarker of Histamine Intolerance: A Retrospective Cohort Study.

Background: Histamine Intolerance (HIT) is a multifaceted pseudoallergic disorder possibly due to defective histamine metabolism. Diamine oxidase (DAO) contributes to histamine degradation and can be measured in the serum. The role of DAO measurement in the diagnostic work-up of HIT still remains unclear, and conflicting results have been reported in the literature. Therefore, we aimed to evaluate the possible clinical usefulness and consistency of DAO value ranges as provided by the assay manufacturer and verify whether they could predict the response to treatment. Methods: We retrospectively analyzed 192 outpatients with HIT symptoms and measured serum DAO values at baseline. Patients were prescribed either with low-histamine diet and/or enzymatic supplementation according to symptom severity and re-evaluated six to eight months later. Patients were stratified into three groups according to DAO levels: <3 U/mL, 3−10 U/mL, and >10 U/mL. HIT severity was assessed on a scale of 1 to 5 before and after treatment. Results: A total of 146 patients completed the study. Gastrointestinal and cutaneous symptoms, often associated with headache, were more frequent in subjects with DAO < 10 U/mL. Symptom severity and DAO ranges were correlated. Patients with intermediate DAO levels (3−10 U/mL) showed a more complex clinical phenotype but also a more significant improvement in symptom severity (score reduction 50%, interquartile range (IQR) = 33−60%) when compared to patients with low DAO (40%, IQR = 20−60%; p = 0.045) or high DAO (33%, IQR = 0−50%; p < 0.001). Complex clinical phenotypes were also more frequent in patients with intermediate DAO levels. Conclusions: HIT is characterized by typical symptoms and low levels of DAO activity. Symptom severity was associated with the degree of DAO deficiency. Patients with DAO values between 3 and 10 U/mL show the best response to treatment (low-histamine diet and/or DAO supplementation). DAO value could arguably be considered as a predictor of clinical response to treatment. Prospective studies are needed to confirm these data.