Antibiotic resistance is one of the most serious medical problems in the world. To counteract the increase in antibiotic resistance, new rapid and effective analytical methods are needed. To effectively treat infections in critically ill patients, optimal antibiotic dosages are required. DrugLog® is an instrument that uses a spectrophotometric method to analyze antibiotics in plasma in the wavelength range 200-800 nm. The aim of this study was to do a method validation of the instrument DrugLog®.     The study material that was used was whole blood from healthy donor and routine citrate plasma samples from the laboratory. The precision of the method and stability of plasma, the best way to filtrate lipids from plasma and four antibiotics (meropenem, cefotaxime, vancomycin, piperacillin/tazobactam) were investigated.     The precision of the method, measured as CV% was less than 0.62 and stability plasma showed a CV% of 135.74 after 24 h in room temperature. The stability for the different antibiotics after 24 h in room temperature showed a CV% of 8.11 for meropenem, 40.80 for vancomycin, 16.55 for cefotaxime and 2.92 for the combination antibiotic piperacillin/tazobactam. It was also determined that bacterial filter was the best way to remove lipids from plasma.     In conclusion DrugLog® is a suitable instrument to analyze concentration of antibiotics in patients during antibiotic treatment, however further validations are needed.