Sir,Urticaria is a common skin disorder, affecting 15%–25% of the population at least once in a lifetime.[1] About 30%–50% of patients with chronic idiopathic urticaria have circulating histamine-releasing autoantibodies to the high-affinity IgE receptor FcεRI on basophiles and mast cells or, less commonly, antibodies to IgE. The term autoimmune urticaria is increasingly being accepted for this subgroup of patients. The autologous serum skin test (ASST) is currently the best in vivo clinical test for detection of in vitro basophil histamine-releasing activity.[2]Husz et al. showed that ASST with undiluted serum gives non-specific positive reactions. Their study suggested the use of 1: 10 and 1: 100 dilutions for the test, in order to avoid non-specific reactions, and also to establish a better diagnostic opportunity for patients with autoimmune urticaria.[3] Pavlovic suggested prozone phenomenon may work in few patients and undiluted serum may show negative test. In some of their patients with chronic urticaria, ASST with undiluted serum was either negative, or dilution of their sera conferred stronger reactivity.[4]ASSTs were performed on 10 patients (6 females and 4 males, age group 20–60 years, mean age 32 years, Duration 6 months to 3 years, mean duration 18 months) with chronic spontaneous urticaria (urticaria with physical causes, urticarial vasculitis, pregnant and lactating mothers were excluded).Antihistamines were stopped for 2 days before performing ASST. From proximal to distal, 0.05 ml samples of undiluted autologous serum, 1:10 and 1: 100 diluted (with 0.9% sterile saline) autologous serum and 0.9% sterile saline were separately injected into the volar aspect of the forearm, with at least 2.5 cm between injection sites. Wheal responses were measured at 30 min. The test was positive if the diameter of the wheals at the site of the undiluted and 1: 10 diluted sera and 1:100 diluted sera injection sites were 1.5 mm larger than it was at the site of sterile saline injection.We found five patients (four females and one male) out of 10 positive for ASST with undiluted sera and with 1:10 diluted sera. Three out of these five patients were positive with 1:100 diluted sera. No patient reacted to diluted sera alone without undiluted sera showing positive result. We found test with undiluted sera most reliable and suitable screening test. Larger studies are required to validate these findings.EAACI/GA2LEN task force consensus report on autologous serum skin test in urticaria do not recommend ASST with diluted sera.[5]