Interpreting Sensitivity Results

In line with new European reporting requirements, the microbiology laboratory report susceptibility results in three categories: S, I or R.

 For some bug-drug combinations, there is no "S" category. Susceptibilities will be reported as either "I" or "R".

·         "S" = susceptible at a standard dosing regimen.

This means there is a high likelihood of therapeutic success using standard doses and dosing intervals.

·         "I" = susceptible at increased drug exposures.

This means there is a high likelihood of therapeutic success if antibiotic exposure is optimised by using higher doses or increasing dosing frequency.
Please see below table for recommended doses and frequency for organisms with "I" susceptibility.

These doses are for adults with normal renal function. For patients with renal impairment, use the maximum safe doses for the patient’s CrCl, discussing with a pharmacist if necessary.

​Organism	​Drug and Dose	​Dosing interval
​ E.coli , Proteus mirabilis,  Klebsiella pneumoniae  and other coliforms (Enterobacterales group)	​Cefuroxime 1.5g IV	​Three times a day
​ Pseudomonas aeruginosa	​Piperacillin tazobactam 4.5g IV  Ceftazidime 2g IV Aztreonam 2g IV Ciprofloxacin 750mg PO Ciprofloxacin 400mg IV	​Four times a day Three times a day Four times a day Twice a day Three times a day
​ Streptococci groups A, C and G And Streptococcus pneumoniae	​Levofloxacin 500mg PO/IV	​Twice a day
​Staphylococcus aureus	​Ciprofloxacin 750mg PO Ciprofloxacin 400mg IV   Clindamycin 450-600mg PO Clindamycin 600-1200mg IV	​Twice a day Three times a day   Four times a day Four times a day
​Haemophilus influenzae	​Amoxicillin 1g TDS PO Amoxicillin 2g TDS IV   Co-amoxiclav 625mg PO prescribed with Amoxicillin 500mg PO   Co-amoxiclav 1.2g IV prescribed with  Amoxicillin 1g IV	​Three times a day Three times a day   Three times a day     Three times a day

·         "R" = resistant.
This means there is a high likelihood of therapeutic failure using this antibiotic.

For paediatrics where susceptibilities are reported as "I" and a dosing range is given in the BNFc, the dose at the higher end of the range should be used.