Lactate measurements aid in the diagnosis and management of sepsis and septic shock. Most major diagnostic test manufacturers (4 of 5 queried) and many laboratories (8 of 10 regional and national labs queried) specifically require plasma lactate collection and transport in fluoride/oxalate (Fl/Ox) tubes on ice or refrigerated, often with expedited handling and separation from cells. These steps are meant to limit in vitro anaerobic glycolysis in the whole blood sample, which would produce elevated lactate results not reflecting the patient condition.
At least two prior studies have convincingly demonstrated that lactate concentrations are stable even at room temperature in Fl/Ox tubes (1, 2). This work suggested that current practice could be updated to simplify pre-analytical requirements. To validate these findings, whole blood was collected in gray-top Fl/Ox or green-top lithium heparin BD Vacutainer plastic tubes and sent to the laboratory on wet ice. Whole blood aliquots were stored at room temperature (23°C) or refrigerated (4°C) for various lengths of time before centrifugation to obtain plasma. Lactate measurements were performed using either Beckman Coulter DxC 800 or Roche Cobas 6000 automated methods. As expected, lactate concentrations increased steadily in the lithium heparin samples (n = 4) with a mean increase of 16.3 mmol/L after 2 days at room temperature. Uncentrifuged whole blood aliquots from Fl/Ox samples (n = 29) stored for up to 3 days at room temperature maintained lactate concentrations within allowable error limits (±0.4 mmol/L or 10%) with a mean increase of 0.1 mmol/L and maximum increase in 3 samples of 0.2 mmol/L.
Refrigerated samples in Fl/Ox tubes (n = 13) stored uncentrifuged for up to 11 days had a mean increase of 0.2 mmol/L and a maximum lactate increase in one sample of 0.5 mmol/L.
Textbooks, reagent manufacturer instructions for use, and tube manufacturer instructions often indicate that Fl/Ox tubes must be transported on ice and stored at refrigerated temperatures. This is important in samples collected for glucose measurements, as fluoride inhibits primarily enolase, one of the last enzymes in the glycolytic pathway, and glucose consumption continues in uncentrifuged blood, if not cooled immediately. However, when enolase is rapidly inhibited in Fl/Ox tubes (1, 2), production of pyruvate by pyruvate kinase is halted, and lactate dehydrogenase is devoid of substrate.
Note that some models of blood gas analyzers are configured for measuring whole blood lactate, and this analyte may be incorporated into blood gas panels for monitoring sepsis patients. However, blood gas syringes generally contain lithium heparin only. Room temperature gas syringes received more than 30 minutes after collection will have significant increases in lactate. In Fl/Ox tubes the lactate is stable at room temperature before centrifugation and testing on an automated chemistry analyzer. Thus the requirement for low temperature transport and storage of lactate samples in Fl/Ox tubes may be removed in order to simplify and expedite sample collection. This may be a useful approach for nursing homes or other outpatient settings where lactate measurements could be helpful for earlier detection of sepsis.
- Astles R, Williams CP, Sedor F. Stability of plasma lactate in vitro in the presence of antiglycolytic agents. Clin Chem 1994; 40(7 Pt 1):1327-30.
- Mikesh LM, Bruns DE. Stabilization of glucose in blood specimens: mechanism of delay in fluoride inhibition of glycolysis [Letter]. Clin Chem 2008; 54(5):930-2.