Our laboratory uses External Proficiency samples which have both HLA -class I and II reactivity. Three samples that were kept at 37-40 deg Celsius in Customs Warehouse for a week due to delay in paper work showed lower HLA -class I reactivity (58%) as compared to Consensus . Reactivity of serum for HLA- class II antibodies was similar to Consensus (>90%). Also the observed specificity for class I was different from Consensus for class I antibodies for two of three samples. I am not able to get any reference on the subject to explain the performance in PT.