Methods
The effect on plasma total carbon dioxide (ctCO2) of collecting a small volume of blood into a large tube was investigated. In a 7-month-old infant presenting with diarrhoea the initial ctCO2 was low when 0·3 mL of sample was received in a 4-mL tube. To investigate this further blood samples were obtained from three volunteers on five occasions and various volumes were placed in a 4 mL container and analysed after varying intervals.
Results
When the sample volume was less than 2 mL there was a significant decrease in ctCO2. A 42% decrease was seen in the tube containing 0·3 mL after 4 h.
Conclusions
We recommend that when a small volume of sample is collected it should be placed in a smaller tube with minimal air space.
