There is marked intra-individual variation in the rate of absorption from an intramuscular dose, volume of distribution and plasma half life, which means that the relationship between dose and concentration is poor. Aminoglycosides show significant toxicity (nephrotoxicity and ototoxicity) at levels just above the plasma concentration required for bactericidal activity. Hence, therapeutic drug monitoring is required for achieving effective therapy and avoiding toxicity. Gentamicin is normally given as a once daily administration (ODA), for this purpose a mid-dose (6-14hrs) sample is taken for serum gentamicin levels.
Sisomicin (>50% cross reactivity) and netilmicin (1.15%) causes significant interference. Samples containing anti-beta-galactosidase antibodies can result in artificially high gentamicin results that do not fit the clinical profile. In very rare cases, human anti-mouse antibodies (HAMA) may cause unreliable results.
Note: Post-dose levels should only be used in patients on multiple daily dosing of gentamicin. Samples should be taken 60 - 90 minutes after dose administered.
For treatment advice, contact Consultant Microbiologist.
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