Effect of sample handling and storage time on the stability of total CO2 in equine plasma.

Reasons for performing study: The stability of total CO2 concentration (ctCO2) in plasma is influenced by storage temperature and handling during sample processing. Conflicting information exists regarding the stability of ctCO2 in equine plasma over time, and the effect of centrifugation on the measured value for plasma ctCO2 is unclear.

Objectives: To determine plasma ctCO2 stability over 5 days when equine blood is collected into Vacutainer tubes, centrifuged within 30 min of collection, and stored at 4 degrees C; and to determine whether a delay in centrifugation increases the rate at which plasma ctCO2 decreases over time.

Methods: Blood was collected from 6 adult horses into 3 ml plastic Vacutainer tubes and randomly assigned to be centrifuged immediately, or after, storage. Plasma ctCO2 was measured in triplicate at 0, 24, 48, 72 and 96 h after collection using a NOVA-4 analyser Data were analysed using multivariable linear regression, with P < 0.05 being defined as significant.

Results: Plasma ctCO2 decreased linearly over time during storage at 4 degrees C. The measured value for ctCO2 decreased at a faster rate (-0.28 mmol/l/day; P < 0.0001) when centrifugation was delayed, compared with immediate centrifugation (-0.10 mmol/l/day). There was a significant effect of sequence of sample analysis on the ctCO2 value when measured in triplicate: the second and third measurements were 0.31 and 0.41 mmol/l lower than the first measurement, respectively.

Conclusions: Blood samples collected from horses into Vacutainer tubes should be centrifuged immediately after collection and analysed as soon as possible to ensure accurate values for plasma ctCO2.

Potential relevance: Failure to centrifuge, or excessive delay in measuring ctCO2 after centrifugation, produces values significantly lower than the true value.