Three audit cycles were undertaken over the same time period each year and included all patients at University Hospitals Bristol, UK, that had gentamicin levels sent to pathology during the time period August to September. This time period corresponds to the period when new junior doctors start working at the trust. Inclusion criteria was any patient receiving once daily intravenous gentamicin using the trust protocol within the given time window.
Prior to the introduction of the online calculator, the patients' ideal body weight (IBW) was calculated, with the aid of reference tables (Appendix
, Figures , ), using the following formulae:
Patients were categorized as obese if their weight was greater than 20% above IBW.
Non-obese patients received a standard dose of 5 mg/kg rounded down to the nearest multiple of 40. In obese patients, a ‘Dose-determining weight’(DDW) was calculated using the formula:
Gentamicin dose was then calculated by multiplying DDW(kg) by 5 mg, then rounding down to the nearest multiple of 40. Reference tables were made available to assist with this calculation (Appendix
, Figure ).
Gentamicin was dosed daily according to this protocol, except in patients with impaired renal function. To determine renal function, creatinine clearance (CrCl) was calculated using the Cockcroft Gault equation:6
If CrCl was 40–59 µmol/l frequency reduced to 36 hourly and to 48 hourly in patients with CrCl 20–39 µmol/l. Patients with CrCl less than 20 were not suitable for Gentamicin.
The protocol required gentamicin levels to be taken after the first dose in all patients. Subsequently, levels were required to be taken after every dose in patients over 65 or with poor renal function, but levels were not required to be checked in patients with normal creatinine clearance provided they were passing adequate volumes of urine.
Following the first audit cycle, an online calculator was produced and published on the hospital intranet. This calculator required the input of a patient's age, sex, weight and serum creatinine, in order to calculate dosage and frequency, and, following the second audit cycle, the prescription chart was altered to advertise this tool and remove dosing formulas.
Each drug dose was assessed retrospectively by one of the authors (JM,ARS,CT,SS,KS). Patients were excluded if there was a documented reason for the prescription not being dosed according to protocol e.g. given according to microbiology advice. Adults were included from all specialties.
The Bristol Royal Infirmary online antibiotic database which was set up to prior to the launch of the antibiotic policy. This recorded specimen processing activity including time of blood sampling, time of arrival in the laboratory, Gentamicin level result and time of level being available. Data for each serum level taken was extracted from this database by the authors (JM,ARS,CT,SS,KS).
On the prescription form it was the responsibility of doctors to calculate dose using patient weight and record creatinine clearance in order to calculate frequency. These calculations were often checked by pharmacists. It was the responsibility of nurses to record the timing of blood samples and record the dose given and the time given. Doctors bore overall responsibility for the correct use of the protocol.
The tests for statistical significance were performed using the SPSS 17.0 statistics program (SPSS Inc. Chicago, IL, USA).