Deterioration of detectable human immunodeficiency virus serum p24 antigen in samples stored for batch testing

Virologic measurements are becoming important surrogate markers for therapeutic efficacy in clinical trials with human immunodeficiency virus (HIV)-infected subjects. One such marker which is inexpensive and easily evaluated is the HIV p-24 antigen. To determine the storage stability of p24 antigen assayed by enzyme-linked immunosorbent assay of serum collected during clinical trials, a retrospective analysis was performed. The p24 antigen results were available from four Adult or Pediatric AIDS Clinical Trials Group protocols: studies 047, 050, 128, and 213. Paired samples (n = 930) which were assayed by ELISA for p24 antigen both in real time and in batch were analyzed for agreement. Batch and real-time values were correlated; however, there was a lack of agreement which increased with prolonged storage time of batched samples and greater p24 antigen levels. The p24 antigen values were significantly lower in the batched samples, which had a maximum storage time of 1,548 days. The degradation rate of p24 antigen per year was 0.052 log10 for samples with less than 30 pg/ml, 0.197 log10 for those with 30 to 100 pg/ml, and 0.245 log10 for those with > 100 pg/ml. Due to degradation over time, use of p24 antigen values from batch assays with long-term storage could bias study results toward a lack of treatment effect. On the basis of these results we make the following recommendations. (i) Samples should be assayed either in real time by laboratories undergoing quality assurance or in batch with short-term storage (less than 1 year). (ii) When real-time assays are to be performed, the serum samples should not be stored at 4 degrees C, but should be frozen immediately after processing and stored frozen until tested.