The effect of long-term storage on measured plasma lactate concentrations and prospective lactate results from a multicenter trial of antiretroviral therapy.

Plasma lactate measurements are typically performed in real time, limiting their usefulness in multicenter or longitudinal studies. To determine the stability of lactate specimens, blood was drawn in sodium fluoride/potassium oxalate tubes from 13 volunteers before and after 5 min of handgrip exercise to intentionally increase lactate concentrations. Plasma was stored at -70 degrees C. Aliquots were assayed in real time and after 1, 3, 6, 9, 12, 18, and 24 months. Real-time lactate concentrations measured at baseline ranged from 0.52 to 2.23 mmol/L before and from 2.91 to 11.04 mmol/L after handgrip exercise. Using a linear mixed model, the estimated change from baseline at month 24 was 1.67% (95% confidence interval, -0.70% to 4.03%) for pre-exercise samples and 0.39% (95% CI, -1.13% to 1.91%) for post-exercise samples. Stored serial specimens from 232 HIV-infected subjects in a multicenter trial of antiretroviral therapy were also assayed centrally. Among those, median plasma lactate increased from baseline to 64 weeks by 0.4 mmol/L with zidovudine+lamivudine treatment and by 0.6 mmol/L with didanosine+stavudine (each p<0.001 from baseline; p=0.04 for difference between groups over time). When performed as in this study, frozen storage with central batch lactate analysis is appropriate for prospectively collected samples in multicenter trials.