How are levels measured in some hospitals?

For many drug therapies, including aminoglycosides, the individual pharmacodynamic effect is not readily measurable. Aminoglycosides have a large inter-individual variation which may translate into differing drug concentrations in the body and differing responses seen in patients. As only a small difference lies between effective and toxic dosing, accurate, personalised dosing is important for patients.

As technology develops, medicine tries to accommodate inter-individual and intra-individual variability in an effort to provide personal dosing strategies and improve clinical outcomes, patient quality of life and reduce toxicity.

Point-of-care testing, which is non-invasive, quick and accurate, would be the ideal solution. However, at present, TDM is still usually carried out within the hospital microbiology or biochemistry laboratories in the UK using specific aminoglycoside test kits, which are processed through an analyser. Usually, drug concentration is monitored from a plasma or serum sample, though whole blood could also be analysed. Sample treatments extract the total drug available in plasma which is made up of the sum of the protein-bound and free drug in circulation. In the UK, there are 6 different methods of assay, and this example explores one such method known as the Turbidimetric method.

How are levels measured in the hospital?

Gentamicin levels are processed through the biochemistry laboratory, which, in some cases, may be fully mechanised. In some hospitals, samples can be analysed 24 hours a day. Some mechanised labs can process more than 3000 samples a day. Locally, 15 – 20 samples/day on average would be for gentamicin blood level monitoring. Blood samples are requested and sent to the biochemistry laboratory, with accompanying information stating the dose, time of last dose and time the sample was taken. This information is very important to be able to understand the result.

Although tests are carried out in the biochemistry laboratory, clinical oversight of the process sits with the microbiology laboratory. A fast turnaround time is important to enable adjustment of dosing of aminoglycosides in ‘real’ time before the next dose is due. Laboratories aim to provide results within one hour if urgent.

Here is an example of how the samples may be processed through a laboratory.

If you require a screen reader-compatible version this is available as a PDF.

Take a closer look at the flow diagram shown above here.

How does the lab know that the gentamicin levels measured are accurate?

Quality control material containing known concentrations of gentamicin are periodically analysed. Providing the results are not more than 3.5% lower or higher than the target mean concentration value, then we are assured that the assay is working properly.

What happens after the results are generated?

All the gentamicin level results are sent directly to the patient’s electronic records. If the results are outside of the recognised ranges, the clinical team will be asked to acknowledge that they have seen the result. Based on the results, clinical teams may then need to act accordingly, however, there can be challenges with interpreting the results and both will be discussed across the next two steps.

Think about the question below, and discuss your answer with other learners in the comments section.

• If you have a TDM service where you work, do you know where this takes place and how the levels are measured?

When you’re done, click next to move on and learn about common issues that can occur when interpreting levels.