Exploring the Role of Autologous Serum Skin Testing in Chronic Urticaria: A Descriptive Study from an Allergy Clinic in Bangalore

Introduction

Chronic urticaria (CU) is a common skin disorder in the community, affecting about 15-20% of the Indian population.¹ It is defined as a condition characterized by the intermittent appearance of hives or wheals persisting for more than six weeks.²

Chronic urticaria is classified based on causes, duration, and frequency. CU is divided into three clinical subgroups, with the spontaneous type being the most common (80%), followed by the physical type (10%) and special forms (10%).³ Chronic spontaneous urticaria (CSU), commonly known as chronic idiopathic urticaria, is characterized by spontaneous occurrence of wheals without any obvious stimulus lasting for more than six weeks.^3,4 As per the published literature, about one-third CSU patients have histamine-releasing autoantibodies directed against the high-affinity IgE receptor or, less frequently, against IgE.^5-7 Many studies have shown that up to 30-50% patients with CSU have associated autoimmune manifestations. Among them, 35-40% of patients present with functional mast cell stimulating IgG antibodies acting against the α-chain of the high-affinity IgE receptor, while 5-10% produce antibodies against IgE itself.^8-14

However, it is often difficult to clinically distinguish chronic urticaria (CU) from autoimmune urticaria (AIU). The autologous serum skin test (ASST) serves as a screening test for autoimmune urticaria.¹⁵ CU can thus be supported in diagnosis by ASST, as patients with chronic urticaria who test positive for ASST typically exhibit functional autoantibodies against IgE and/or its high-affinity receptor FceRI. The diagnosis of CU is mainly based on clinical suspicion and ASST, and facilities for assaying functional histamine release from basophils or mast cells.¹⁶

Autologous serum skin test (ASST) is a simple in vivo clinical test based on the principle of detecting basophil histamine release activity. The sensitivity and specificity of ASST are approximately 70% and 80%, respectively, making it a reasonably accurate predictive clinical test for the presence of functional circulating autoantibodies.¹⁷ Hence, ASST can be considered a rapid, reliable in vivo test to help distinguish between chronic urticaria and autoimmune urticaria.

This study was undertaken with the following objectives: to describe the sociodemographic profile of the study subjects and to estimate the proportion of positive autologous serum skin test results among patients with chronic urticaria.

Materials and Methods

This was a prospective descriptive study undertaken at an allergy centre attached to a medical college in Bengaluru. The study was conducted from January 2022 to June 2022, over a period of six months. The sample size was calculated using the Epi Info Program (Version 7.4.2), with the power of study set at 80 percent, a confidence interval of 99 percent, and a point prevalence value of exposure considered as 1 percent.

A total of 208 consenting subjects who met the study’s inclusion and exclusion criteria were enrolled by continuous sampling, with confidentiality maintained. ASST was performed for all participants. The inclusion criteria was patients with daily or almost daily occurrence of urticarial wheals for a period of six weeks or more. The exclusion criteria constituted pregnant and lactating women, patients with urticarial vasculitis, patients with known drug allergies, those receiving corticosteroid therapy, and severely ill patients. A standard case record form was used for data collection. Information regarding the sociodemographic profile, clinical symptoms, signs and other relevant details was recorded.

A detailed clinical history regarding possible aggravating factors of urticaria was collected. This included information on the time of onset of symptoms, duration of individual wheals (in hours), overall duration of disease (in months/years), diameter and distribution of wheals, and associated angioedema. A general and relevant dermatological examination was also performed. All patients were instructed to discontinue antihistamines for 5 to 7 days prior to undergoing ASST in order to avoid false-negative results.^18,19

Finally, ASST was performed as per the standard guidelines under aseptic precautions. From the cubital vein, approximately 5 mL of venous blood was drawn and collected in a sterile vacutainer, following all standard precautions, and allowed to clot at room temperature for 30 minutes. The blood sample was centrifuged at 3000 rpm for 15 minutes. Approximately 0.1 mL of serum was intradermally injected into the left forearm, about 2 cm below the cubital fossa, using a 1 mL insulin syringe with a 30-gauge needle, in an area of uninvolved skin. Similarly, 0.1 mL of 0.9% sterile normal saline was injected intradermally into the right forearm to serve as the negative control. The test and control arms were kept separate to ensure uniformity in the study protocol, for the sake of convention, ease of administration of injections, result interpretation, and to avoid any confusion.²⁰ The test was considered positive if the erythematous wheal induced by the serum was ≥ 2 mm larger in diameter than the saline induced response, 30 minutes after administration.

Data were entered in Microsoft Excel and analysed using OpenEpi. Descriptive statistics like mean, standard deviation, frequency, and percentages were used to present the study results, while the Chi-square test was applied for analyzing inferential statistics.

Results

A total of 208 subjects were included in the study population. The average age of the subjects was 38.36 years, with a standard deviation of +13.35 years. Of the 208 subjects, 105 (50.5%) were males and 103 (49.5%) were females. Majority of the subjects were in the reproductive age group. Most subjects were Hindus and 75 (36.05%) belonged to the upper middle class, as per Modified Kuppuswamy’s classification.

Most of the subjects, 97 (45.2%), presented with itching of less than one year’s duration, and most of the symptoms persisted throughout the year. Rashes with itching were the main presenting symptom in 182 (87.5%) participants. Angioedema was reported in 33 (15.9%) subjects, while dermographism along with itching and rashes was noted in 36 (17.3%) (Table 1).

Out of 208 patients, 131 (63%) tested positive for ASST, of which 62 (47.3%) were males and 69 (52.7%) were females. No significant association was noted between gender and positive ASST test (Table 2).

Discussion

The present study aimed at assessing the relationship between chronic spontaneous urticaria (CSU) and autologous serum skin test (ASST) among the patients attending an allergy clinic in Bangalore. The study also sought to describe the sociodemographic characteristics of the patients and estimate the proportion of positive ASST results in this cohort.

In line with previous studies, the majority of participants were from the reproductive age group, with an average age of 38.36 years.²⁰ This is consistent with existing literature, which indicates that chronic urticaria is more prevalent in adults, especially those aged between 20 and 50 years.¹ The relatively equal gender distribution observed in this study (50.5% males and 49.5% females) aligns with findings from previous studies, where the incidence of CSU is often reported to be similar across both sexes, although some studies have suggested a slight female predominance.²¹

In the current study, the clinical presentation of CSU was in agreement with the characteristic symptoms of the condition, with the majority of patients reporting itching and rashes (87.5%) as the predominant symptoms. This finding is in concordance with the previous studies which identified pruritus (itching) and recurrent appearance of hives or wheals as the most common manifestations of CSU.²² A small proportion of patients (15.9%) presented with angioedema, which is a well-known feature of CSU, though not universally present in all cases.²³ Additionally, a significant number of subjects (17.3%) exhibited dermographism, which is another common feature associated with chronic urticaria, reinforcing the heterogeneous nature of the disease's clinical presentation.

One of the primary objectives of the study was to estimate the proportion of ASST positivity in CSU subjects. The results revealed that 63% of the subjects tested positive for the autologous serum skin test (ASST), which is in concordance with the findings of several studies, indicating that a significant proportion of CSU patients exhibit positive ASST results.²⁴ This reinforces the notion that the presence of functional autoantibodies against IgE or its high-affinity receptor (FceRI) plays an important role in the pathophysiology of CSU.^5,20,24 Previous studies have reported positivity rates ranging from 30% to 50%, with variations attributed to differences in study methodology and the population studied ^6,7,19

The finding that 52.7% of females and 47.3% of males tested positive for ASST shows no significant gender-based difference in the test results, which aligns with the findings of other research studies suggesting that gender may not significantly affect the likelihood of a positive ASST in CSU patients.^10,22,23 Furthermore, the absence of a significant association between gender and ASST positivity in this study suggests that both males and females with CSU have a similar likelihood of autoimmune involvement in their condition.

The high rate of positive ASST in this study underscores the importance of the test as a diagnostic tool in the assessment of autoimmune involvement in CSU. As an easy, reliable, and inexpensive test, ASST provides an in vivo means of detecting circulating autoantibodies and is thus an essential tool for identifying autoimmune urticaria, which accounts for a significant subset of CSU cases.^17,22,25 In clinical practice, ASST serves as an essential screening test, especially when the diagnosis of CSU is uncertain and distinguishing autoimmune urticaria from other forms of chronic urticaria is necessary.²⁰

However, while the ASST has a high sensitivity (approximately 70%) and specificity (around 80%), it has its own limitations. The false positive and false negative results can occur, and a positive ASST result alone is not sufficient for a definitive diagnosis. A comprehensive clinical assessment, including a detailed history and physical examination, is essential for accurate diagnosis and appropriate management of CSU. Additionally, a positive ASST result does not always correlate with clinical severity, which emphasizes the need for careful interpretation of results in the context of the individual patient's clinical picture.

Conclusion

To conclude, the present study provides valuable insights into the sociodemographic and clinical profile of chronic spontaneous urticaria patients attending an allergy clinic attached to a medical college in Bangalore. The high ASST positivity rate (63%) in the study population supports the notion that autoimmune mechanisms play a significant role in the pathogenesis of CSU. The findings of the study also highlight the utility of ASST as a reliable screening tool for identifying autoimmune urticaria. Future studies with larger sample sizes and longitudinal follow-up may further elucidate the long-term outcomes of CSU patients with positive ASST results and explore the potential for more personalized therapeutic approaches in managing autoimmune urticaria. 

Limitations

This study has certain limitations, including a relatively small sample size. In addition, in vitro tests were not done for comparison and validation. Other potential markers, like antithyroid antibody levels (anti-TPO), D-dimer, and IL 6, were also not assessed.

Conflict of interest

Nil