Stability of diluted chlorhexidine for skin testing in drug allergy evaluations

Background: Chlorhexidine gluconate (CHX), a common cause of perioperative anaphylaxis, is frequently used for skin testing in allergy evaluations. Although CHX's maximal nonirritating concentrations are known, the stability of its dilutions for skin testing remains unexplored, particularly when sterile water for injection (SWFI) or normal saline (NS) are used as diluents. Objective: Our aim was to evaluate the stability and precipitation of CHX when diluted with SWFI or NS for drug allergy skin testing. Methods: CHX dilutions (5-0.002 mg/mL) were prepared using SWFI and NS. HPLC and UV-visible spectrophotometry were used to assess stability and precipitation over 48 hours. Turbidity was measured at various time points to monitor precipitation. Results: HPLC analysis showed no significant differences in peak heights between CHX-SWFI and CHX-NS dilutions. However, visible precipitation and increased turbidity (>100 NTU) were observed in CHX-NS at higher concentrations (5 mg/mL) after 60 minutes. No precipitation occurred in CHX-SWFI at any concentration for 48 hours. Conclusion: For CHX skin testing, SWFI is the preferred diluent at concentrations higher than 0.02 mg/mL to avoid precipitation. Using NS for the final dilution from 0.02 to 0.002 mg/mL is feasible and reduces injection pain. Except for CHX-NS at 5 mg/mL, reagents can be prepared up to 24 hours before testing.