General Information
Determination of serum antibiotic concentration is usually necessary in two circumstances:-
- Where drugs of known, dose-related toxicity are used.
and
- b) In order to monitor levels when the drug used has a narrow therapeutic range.
For aminoglycosides (gentamicin and tobramycin), efficiency is related to peak concentrations achieved, while for glycopeptides such as vancomycin, efficiency is related to the time over which therapeutic levels are maintained.
Information on performing antimicrobial assays is given below, but Medical Microbiologists are available, at all times, to discuss assays and the interpretation of results.
Antibiotic dosage in renal failure can be obtained from the appropriate Intranet site.
Antibiotic assays are performed twice a day and are processed in batches, a morning run and an afternoon run. Any bloods sent for antibiotic assays after 2pm will be processed the next day.
Assays are NOT generally available out of hours, but blood may be taken and stored on the ward to be submitted the following morning.
All of these antibiotics are stable at room temperature or in the refrigerator.
Samples For all antibiotic assays, 5-10mls blood into red-topped tube (no anticoagulant) DO NOT sample from the line through which the antibiotic has been given.
| Antibiotic | Sampling Time | First Sample | Expected Levels | Usual interval to re-assay
|
| VANCOMYCIN
|
Pre-dose only (i.e. within 15 mins before dose)
|
Around 2nd – 4th dose |
5 – 15mg/L |
3 days
|
| GENTAMICIN / TOBRAMYCIN (b.d or t.d.s regime) |
Pre-dose (within 15 mins before dose) and Post-dose (1hr after dose) |
Around 2nd – 4th dose |
Pre less than 2mg/L Post 5 – 10 mg/L |
3 days |
| GENTAMICIN / TOBRAMYCIN (once – daily regime) | Post-dose only taken between 6 and 14 hours after dose.
N.B. Times of dosing and sampling must be given on request form. |
1st to 3rd dose |
Interpretation by Microbiologist |
3 days |
Important Notes
- After taking pre-dose levels, give the prescribed dose as normal – DO NOT wait for the results of the assay.
- “Random” levels should seldom be needed and should only be taken by prior arrangement with the Duty Microbiologist.
- For patients with stable renal function, whose results are within the expected range, assays do not need to be performed more than twice a week. Where renal function is unstable or poor, assays should be discussed with the Duty Microbiologist.
General notes: Blood is normally sterile and the presence of bacteria in the bloodstream (bacteraemia/septicaema) is a serious & potentially life-threatening condition. As even very small numbers of bacteria in the blood can cause severe illness, enough blood must be sampled to give the lab a chance of detecting them. Similarly, the techniques used to detect bacteria in blood are designed to pick up tiny numbers. This means that unless the samples are taken with great care & flawless aseptic technique, contaminants from the skin of patients or staff, cannulae or environment may be inoculated with the blood and give a positive result on culture.
It is often very difficult to distinguish contaminating bacteria from real pathogens and, in the best interests of the patient, each positive blood culture has to be telephoned to the relevant clinical team, by a microbiology registrar or consultant in order to assess the clinical implications. Often, patients have to be given antibiotics as a precaution, while a full identification of the bacterium is carried out.
Procedure
Is available on the HEFT Infection control site : Procedure for taking Blood Cultures
Molecular Bacteriology
Molecular bacteriology is a relatively new, quickly-evolving laboratory. Current tests available are Rapid MRSA PCR, Clostridium difficile Ribotyping, Clostridium difficile PCR and VTEC O157 PCR. Please search for these tests in the test database for more information on sample type.
Lead BMS: Priti Rathod
Please note that VTEC O157 requests require a specific request form. Please click here for a copy.
