Analysis of blood gases and blood pH yield important information in many situations of clinical emergencies. We report on a patient in whom pre-analytic errors in blood gas and blood pH measurements resulted in unnecessary further investigations. We therefore studied various pre-analytic sources of error in blood pH and blood gas analysis. Delay in sample processing for more than one hour resulted in an increase of pO2 and pCO2 and a decrease of pH. Excess sodium heparin solution as an anticoagulant (> or = 10% of total volume) led to a significant decrease of pH and pCO2 and to an increase of pO2. Air bubbles (10% of total volume) left in the syringe for 10 min significantly increased pO2. For accurate estimations of pO2, pCO2 and pH, it is necessary to keep the heparin solution below 10% of total volume, to expel all air bubbles from the syringe and to process the blood sample within one hour. Instructions to medical staff on handling blood samples for blood gas analysis should include these possible sources of errors.