title: A microbiological method for determining serum levels of broad spectrum β-lactam antibiotics in critically ill patients.
abstract: Recent studies show that suboptimal blood levels of β-lactam antibiotics are present in intensive care unit (ICU) patients. A common reference method for assessing drug concentrations is liquid chromatography coupled with mass-spectrometry (LC-MS) which is highly accurate but rarely available outside reference centres. Thus, our aim was to develop a microbiological method for monitoring β-lactam antibiotic serum levels which could be used at any hospital with a microbiological laboratory.
  
  
  
  The method was developed as a 96-well broth microdilution format to assess the concentrations of cefotaxime (CTX), meropenem (MER), and piperacillin (PIP). Patient serum containing antibiotics were diluted in suspensions of bacteria with known minimal inhibitory concentrations (MICs). Serum antibiotic concentrations were calculated by dividing the MIC with the dilution factor at which the serum inhibited growth of the bacterial suspension. Serum (n=88) from ICU patients at four hospitals in south-east Sweden were analysed and compared to LC-MS analysis.
  
  
  
  The overall accuracy and precision for spiked samples and patient samples was within the pre-set target of ±20.0% for all drugs. There was a significant correlation between the microbiological assay and LC-MS for the patient samples (CTX: r=0.86, n=31; MER: r=0.96, n=11; PIP: r=0.88, n=39) and the agreement around the clinical cut-off for CTX (4.0mg/l), MER (2.0mg/l) and PIP (16.0mg/l) was 90%, 100% and 87%, respectively.
  
  
  
  The microbiological method has a performance for determination of serum levels of meropenem, piperacillin and cefotaxime suitable for clinical use. It is an inexpensive method applicable in any microbiology laboratory.
authors.affiliation: Department of Clinical Microbiology, Kalmar County Hospital, Kalmar, Sweden.
authors.forename: Jimmy
authors.initials: J
authors.lastname: Fridlund
authors.affiliation: Department of Clinical Microbiology, Kalmar County Hospital, Kalmar, Sweden; Department of Medicine and Optometry, Linnaeus University, Kalmar, Sweden.
authors.forename: Hanna
authors.initials: H
authors.lastname: Woksepp
authors.affiliation: Department of Clinical Microbiology, Kalmar County Hospital, Kalmar, Sweden; Department of Medicine and Optometry, Linnaeus University, Kalmar, Sweden; Department of Medical Microbiology, Linköping University, Linköping, Sweden. Electronic address: tschon@hotmail.com.
authors.forename: Thomas
authors.initials: T
authors.lastname: Schön
mesh_terms: D000900:Anti-Bacterial Agents / Q000097:blood* / Q000494:pharmacology; D001419:Bacteria / Q000187:drug effects* / Q000254:growth & development; D001431:Bacteriological Techniques; D002853:Chromatography, Liquid; D016638:Critical Illness; D006801:Humans; D013058:Mass Spectrometry; D000077731:Meropenem; D008826:Microbial Sensitivity Tests; D013548:Sweden; D013845:Thienamycins / Q000097:blood / Q000494:pharmacology; D047090:beta-Lactams / Q000097:blood* / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D013845:Thienamycins; D047090:beta-Lactams; D000077731:Meropenem
keywords: Cefotaxime; Drug concentration; Meropenem; Microbiological method; Piperacillin; β-Lactam antibiotics