The purpose of this study is to describe a low-cost and simply made instrument capable of measuring the total CO 2 content of microliter volumes of biological fluids utilizing a commercially available CO 2 sensor based on a NDIR detector. The described instrument is based on transformation of dissolved HCO 3 − to CO 2 by acidification and subsequent measurement of the produced CO 2. The instrument has a linear response in the range 0.025–10 μmol HCO 3 −, which enables measurements in fresh urine and plasma samples down to 5 μl. The values from plasma were compared to measurements made on 65 μl whole blood in an automatic blood gas analyzer and found not to differ significantly. Compared to currently commercially available instruments applying the same principles to measure total CO 2, this study provides a simple and robust alternative which even can be used on smaller sample volumes.