Do patients with local reactions to allergy shots require dosage reductions for subsequent injections?

BACKGROUND: Many physicians reduce the dose of allergen immunotherapy when patients have significant local reactions to their allergy shots, believing that these patients are at higher risk for systemic reactions. This dose reduction is made despite the fact that the World Health Organization stated in a position paper on allergen immunotherapy that local reactions are not predictive of subsequent systemic reactions.

POPULATION STUDIED: This study was conducted at a single-site Air Force allergy clinic. During the 18-month study period 12,926 allergy shots were given. No further demographic details were provided.

STUDY DESIGN AND VALIDITY: This nonconcurrent cohort study compared reaction rates to allergy shots for 9 months (October 1996 to June 1997) before an intervention with reaction rates for the 9 months (October 1997 to June 1998) after the intervention. The first group (8076 injections) had their allergy shot dose reduced if they had an immediate local reaction 20 mm or larger or if they had any localized swelling that persisted more than 12 hours. The second group (4850 injections) had no dose reduction for immediate and local reactions unless the reaction was larger than the patient’s hand (adult=8-10 cm) or caused the patient significant discomfort. In most respects the study groups can be considered similar. In fact, in many instances the same subject was probably included in both groups (because most patients receive allergy shots for several years they would have been captured in both 9-month study periods). The potential for differences in the study groups comes from selection bias and those lost to follow-up. The first 9-month period included 8076 injections, while the second 9-month period had only 4850 injections. The authors state that this is because there was difficulty getting extract during the second 9-month period, which delayed the initiation of immunotherapy for some. Because allergy shots can be grouped into 2 phases (build-up and maintenance) and the traditional teaching is that reactions are less common in patients getting maintenance shots, the second group may have had a higher proportion receiving the less-risky maintenance injections. The follow-up of both groups was by review of clinic records, of which 74% were located for the first group and 78% for the second group. Bias could be introduced if the patients who were lost to follow-up were significantly different.

OUTCOMES MEASURED: Systemic reaction rates during the 2 periods were determined. Among those with a systemic reaction, the number of times a local reaction immediately preceded the systemic reaction and the total number of previous local reactions were also determined.

RESULTS: Systemic reaction rates were not statistically different during the 2 9-month periods (0.8% before vs 1.0% after, P=.24). The number of times a local reaction preceded a systemic reaction in the first period was not significantly different from the second 9-month period (18.8% before vs 10.5% after, P=.37). The total local reaction rate for those with systemic reactions was not significantly different during the 2 study periods (7.3% before vs 4.7% after, P=.07). The calculated sensitivity for a local reaction predicting a systemic reaction at the next dose was 15% with a positive predictive value of a local reaction for a subsequent systemic reaction of 17%.