The purpose of this study was to examine changes in levels of biochemical markers such as glucose, blood urea nitrogen (BUN), creatinine, sodium (Na+), potassium (K+), chloride (Cl-),

Total CO2 (TCO2) in blood serum after centrifugation and standing the blood collecting tubes at room temperature (25 ๐C) for up to seven hours. In this experimental and comparative study, the 120 samples were collected from patients who underwent examination in the Clinical Chemistry Unit, Department of Pathology, Songklanagarind Hospital, Faculty of Medicine, Prince of Songkla University. The blood samples were centrifuged and analyzed within 30 minutes after collecting and left standing for 1, 3, 5 and 7 hours at room temperature. After standing, the samples were analyzed again. The results showed that the mean averages of serum BUN and creatinine in blood samples left to stand for 7 hours were not different from the immediately measured samples. The mean average of serum potassium in blood samples left to stand for 1 hour was not different. After 1-hour standing, the means of serum glucose, sodium, chloride and Total CO2 were signifi-cantly different from the immediately measured blood samples (p<0.05). However the average changes of all chemicals after centrifugation and standing at room temperature for up to seven hours were not exceed the clinical acceptance limit and the allowable error limit of these tests.