3) Recognize interfering substances as potential causes of laboratory errors. CASE INFORMATION: A 72 year-old male was admitted for respiratory distress and confusion, and found to have a right upper lobe lung mass and hypercalcemia. He was intubated on the second hospital day due to worsening of his respiratory status. Propofol was initiated for sedation and methylprednisolone and levofloxacin were given for a presumed postobstructive pneumonia. His initial arterial blood gas after intubation showed a pH 7.38 pCO2 38 pO2 143 on an Fio2 of .60. His measured HCO3 was 26 meq/l. Over the next 4 days, his measured bicarbonate progressively dropped to 8 meq/l despite no change in his arterial blood gas (pH 7.38 pCO2 36 pO2 103 on an FiO2 of .45). His anion gap was calculated at 19. Serum lactate was normal, and serum ketones were absent. Consultation with the pathology department revealed the patient's serum to be grossly lipemic. A review of the chart revealed the patient did receive lipid infusions with TPN 36 and 18 hours prior to this discovery. A lipid panel was obtained and revealed marked hypertriglyceridemia at 4426 mg/dl. The lipid infusions were discontinued, and the propofol was weaned off. The bicarbonate level dropped to a low of 3 meq/l approximately 7 hours after the medication was discontinued. Four hours later, the bicarbonate had corrected to 21 meq/l. The serum, however, remained grossly lipemic. The patient's condition continued to decline with the development of septic shock, multi-organ failure and ventricular arrythmias. Results of a previous bronchoscopy demonstrated small cell carcinoma. The patient's family requested no further aggressive treatment and he expired later that day. IMPLICATIONS/DISCUSSION: This patient developed marked derangement in his measured bicarbonate levels that did not correspond to his arterial blood gas analysis. A laboratory error was hypothesized as the cause. Due to the finding of lipemic serum, the hypertriglyceridemia was initially suspected as the interfering substance. Upon discontinuing the propofol, the serum bicarbonate level normalized, the serum, however remained lipemic. In a review of the literature, neither propofol nor hypertriglyceridemia have been reported as potential causes of this lab error. Further testing is needed to determine the role of propofol as an interfering substance in bicarbonate laboratory analysis.