Chronic spontaneous urticaria (CSU) is a common and debilitating disease, affecting adults and children, with a marked impact on patients’ quality of life (1–5). It is defined by the spontaneous appearance of transient itchy wheals (hives), angioedema, or both for more than 6 weeks (1–6). Recent advances have characterized CSU as an autoantibody-driven disease, where mast cells and basophils in the skin are activated through two distinct pathways (4, 5).

In type I autoimmune CSU (TIaiCSU, also called autoallergic CSU), IgE autoantibodies are cross-linked by self-antigens, for example thyroid peroxidase or interleukin 24 (2–5). In type IIb autoimmune CSU (TIIbaiCSU), IgG and IgM autoantibodies are directed against IgE receptors (or IgE itself) on the surface of mast cells and basophils (1–7).

In clinical practice, it is important to know if a patient has TIIbaiCSU, as TIaiCSU and TIIbaiCSU present distinct phenotypes. Patients with TIIbaiCSU have been shown to have an increased risk of developing other autoimmune diseases, of failing antihistamine and omalizumab treatment, and of having a better response to cyclosporine and fenebrutinib (8–11). They have also been linked to higher disease activity and impact, making effective treatment both critical and challenging. The identification of further characteristics that characterize both subgroups is still a matter of ongoing research (4).

TIIbaiCSU is diagnosed by the combination of three tests, i.e. the autologous serum skin test (ASST), the basophil activation test (BAT) or the basophil histamine release assay (BHRA), and an ELISA or Western Blot-based autoantibody assay (1). Testing of all CSU patients for TIIBaiCSU with these three test is difficult, for several reasons, including their availability and the high and increasing prevalence of CSU, of up to 1.4% (12).

Basophil testing, with the BAT or the BHRA, is the single best diagnostic test for TIIbaiCSU. The recent PURIST study found that the BAT and the BHRA were 69% and 88% predictive of TIIbaiCSU, respectively (4). In addition, several reports show that the BAT/BHRA alone can identify patients with higher disease activity, longer disease duration, and poorer response to omalizumab (1, 9, 13, 14). In contrast, the ASST, initially thought off as a good, inexpensive, globally available marker of TIIbaiCSU, is only 27% predictive of TIIbaiCSU (4). Moreover, the ASST is influenced by the intake of antihistamines (the first-line treatment and a treatment difficult to interrupt in patients with severe urticaria), it does not give a quantifiable result, and handling of biologic samples requires exceptional care, as reinjecting patients’ sera has important safety requirements (15).

Basophil testing of patients with CSU is performed at many urticaria centers of reference and excellence [UCAREs (16)] and by a limited number of commercial vendors. Finding ways of selecting patients who benefit the most from performing TIIbaiCSU tests are needed; as diagnosing CSU patients with TIIbaiCSU has real implications for the patient and the physician.

The present CORSA (component-resolved screening for autoimmune chronic spontaneous urticaria) study has two main objectives: 1) to confirm known and identify as of yet unknown features of BAT/BHRA-positive CSU patients, 2) to identify combinations of clinical and laboratory markers of CSU patients that can help to guide patient selection for basophil testing.

The results of the CORSA study show that basophil test-positive patients (either BAT or BHRA) represent a distinct subset of CSU patients, with characteristics of TIIbaiCSU detailed by the recent PURIST study (4).

In our cohort, 44% and 28% of the patients were BAT-positive and BHRA-positive, respectively. This this is expected, as most studies report basophil test-positive results in 25% to 45% of the cases (8, 9, 23–25), with some studies going as high as 64% (26). However, even though our BAT and BHRA tests showed significant correlation and a 70% match in the results, this correlation was not perfect. The precise reason for this mismatch is currently unknown, however, not unexpected.

There are few studies directly comparing the two basophil tests in CSU. A previous study by our Department evaluated the correlation between ASST and BAT positivity in 48 CSU patients, and found a correlation coefficient of 0.79 (27). A study by Altrich et al. calculated a correlation coefficient between both tests of r=0.54, even though the concordance (positive versus negative) was high (75%); which is similar to the results presented in this study (28). Another study, by Yasnowsky and co-workers, showed a correlation of r=0.6, although in this case the BAT test was evaluated using CD203c expression, which may have influenced these results (29). Szegedi and colleagues published results with a higher degree of correlation: r=0.91 when an atopic donor was used and r=0.7 when a non-atopic donor was used (30). However, these results highlight an important caveat; the correlation between basophil test results can change significantly with the characteristics of the donor of the basophils used. Additionally, in Szegedi’s study, the same atopic and non-atopic donor were used for all the tests. In our study, different donors were used for the BAT and the BHRA, which can further explain the correlation discrepancies.

Not only do the BAT and BHRA use different techniques, which can impact the degree of basophil activation, it is hard to quantify the impact of using different basophil donors. This is also true for the tests individually. In the BAT and BHRA, the same serum tested at the same time with different donors, produces different degrees of basophil activation. The basophil test (BAT and BHRA) and ASST mismatch, on the other hand, was expected and has been described in the literature (24, 28–32).

The recent PURIST study, the first study to identify TIIbaiCSU patients using all three defining tests, provided important evidence of the distinctive characteristics of these patients. It showed that this patient subgroup presented higher disease activity (i.e. UAS7), lower total serum IgE, higher rates of thyroid autoantibodies and a higher IgG-anti-TPO to total serum IgE ratio (4). The results of the CORSA study show that our basophil test-positive patients exhibit a phenotype that is distinct from that of our basophil test-negative patients, and which is in agreement with the TIIbaiCSU profile of the patients of the PURIST study. Our basophil test-positive patients presented higher disease activity (UAS7), higher disease impact (DLQI), lower disease control (UCT), lower total serum IgE, higher rates of thyroid autoantibodies and a higher IgG-anti-TPO to total serum IgE ratio.

However, these results were based solely on basophil testing, one of the three tests required to identify TIIbaiCSU. Therefore, we re-analysed the phenotypes after adding the ASST (a second criterion for TIIbaiCSU). The results remained unchanged. This suggests that the basophil test is the more significant test for identifying patients with this more severe phenotype. In fact, most previous studies, which reported higher disease severity (24, 25), lower disease control (11), elevated thyroid autoantibodies (9, 14, 28), a non-response or slower response to omalizumab, and a better response to cyclosporin (8–11), based their findings on a single basophil test (BAT or BHRA) and not on all three tests needed to identify TIIbaiCSU. This makes the case for why basophil testing is so important for evaluating CSU patients for type IIb autoimmunity and how it can help to identify cases of severe CSU early on.

Using both the BAT and the BHRA, we were also able to identify novel clinical characteristics of basophil test-positive patients. The CORSA study basophil test-positive patients showed a higher frequency of angioedema, nocturnal symptoms, and wheals on five or more days per week; characteristics which may have influenced the poorer results on the UAS7, DLQI and UCT scores.

Despite its relevance, basophil testing meets real challenges as a solution for widespread implementation for all CSU patients, namely in terms of its availability, cost, and the high prevalence of CSU in the population. Identifying markers of basophil test-positive patients would be of great use in clinical practice, as they could select the best group of patients in whom to perform the test.

The ASST, the first attempt at such a marker, proved insufficient (1, 4). Our results confirm this and show a sensitivity of only 53% at identifying BAT+ and/or BHRA+ patients. In an attempt to improve these results, we used combinations of the ASST with the characteristics we identified in our basophil test-positive patients. ASST-positive patients with simultaneous presence of thyroid autoantibodies or a total serum IgE<30, showed a ≈100% probability (specificity) of having a positive basophil test. The combination of total serum IgE<30 and the presence of thyroid autoantibodies also showed 100% specificity. However, these combinations showed a very low sensitivity, and would leave out more basophil test-positive patients than the ASST.

A stepwise approach, employing a decision tree, using the characteristics associated with basophil test positivity, correctly identified most of our patients: it correctly identified 33 of 48 basophil test-positive patients and 27 of 37 basophil test-negative patients. This decision tree is easy to use and easily replicable and can be a promising tool to many physicians treating CSU patients. By this model, restricting the patients tested with a basophil test to those indicated, a physician could: 1) significantly reduce the number of patients in whom to ask for a basophil test to a cost-effective number, and 2) increase the number of positive basophil tests from the 25-45% average described in the literature for the whole CSU population, to about 70%.

This study has some limitations. It has a limited number of patients, from a single center, and these findings were not tested in a control group. There is a need for further studies to determine why TIIbaiCSU or a basophil-positive patient presents with more severe CSU, lower IgE, and higher thyroid autoantibodies. In addition, these findings need to be corroborated by future studies in larger and more diverse patient populations, and the proposed decision tree should be looked at as an initial attempt to identify basophil-positive patients, to be perfected by future studies. The identification of additional easily available clinical/laboratory parameters and their insertion in the model might be useful to increase the specificity and sensitivity of this algorithm.

In conclusion, basophil testing identifies a distinct subset of CSU patients, consistent with TIIbaiCSU. Using clinical and laboratory characteristics of TIIbaiCSU, it is possible to identify patients who are likely to have a positive basophil test, allowing for a more patient-specific approach to CSU patients in the future and a more cost-effective use of the basophil tests.

The datasets presented in this article are not readily available because it requires authorization by ethics committee. Requests to access the datasets should be directed to corresponding author JM.

The studies involving human participants were reviewed and approved by Comissão de Ética do Centro Hospitalar Universitário Lisboa Norte e Centro Académico Médico de Lisboa (Ethics Committee authorization references 129/17 and 339/19). The patients/participants provided their written informed consent to participate in this study.

JM, KB, PS, and MM contributed to conception and design of the study. JM, PS, and KB organized the database, performed the statistical analysis. JM and MM wrote the first draft of the manuscript. KB, PS, MS, IW, JS, SA, AW, CC, and MP-B wrote sections of the manuscript. All authors contributed to the article and approved the submitted version.

All of the costs for this study were supported by our institutions, without external funding.

KB is employed by RefLab ApS. PS is acting as scientific advisor for RefLab and EP Medical. MM is or recently was a speaker and/or advisor for and/or has received research funding from Allakos, Aralez, AstraZeneca, CSL Behring, FAES, Genentech, Menarini, Novartis, Leo Pharma, Lilly, Moxie, MSD, Roche, Sanofi, UCB, and Uriach. SA is or recently was a speaker and/or advisor for and/or has received research funding from Allakos, AstraZeneca, CSL Behring, Novartis and Moxie.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling Editor declared a past co-authorship with one of the authors MM.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.