Important information for all GPs : Changes to Microbiology reports
Background
The pathology laboratories of Barts Health, Lewisham & Greenwich and Homerton University Hospital NHS Trusts have formed a partnership; the East and South East London Pathology Partnership (ESEL). In the formation of ESEL we have reviewed and aligned the way we process samples and report Microbiology results. From Monday 5th July 2021 there will be several minor changes and two major changes in the reporting of some Microbiology results.
Minor changes will be noticed in the reporting comments and phrases used by the laboratory. These will not directly affect the result, but should lead to standardisation of Microbiology report comments. For example “No Significant growth” will be used in place of phrases such as normal / vaginal / upper respiratory / skin / throat flora.
One of the major changes is that ESEL will adopt the antibiotic susceptibility testing and reporting guidelines from the European Committee on Antimicrobial Susceptibility Testing (EUCAST). As part of this change, service users will notice that some susceptibility test reports will appear with the letter ‘D’.
The second major change affects the format of some quantifiable urine parameters. Both of these major changes are outlined below:
- Antibiotic susceptibility result changes
The European Committee on Antimicrobial Susceptibility Testing (EUCAST) have updated their guidance on the reporting of antibiotics that are deemed susceptible or resistant. They will now be reported as
- “R” = Resistant
- “S” = Susceptible at standard dose
- “D” = Dose Dependent”e. susceptible but only at a higher than standard dose. In most cases this replaces the term “I” or “intermediate” susceptibility
Explanation
Antimicrobial susceptibility testing guides clinicians in their choice of antimicrobial therapy. Results of susceptibility testing are reported when there is a reliable correlation between the results of in vitro susceptibility tests and clinical outcome.
Since 2002, EUCAST has used the terms ’susceptible’(S), ‘intermediate’ (I) and ‘resistant’ (R) to classify the likely outcome if the organism is treated with a specific antibiotic.
Susceptibility tests reported as ‘intermediate’ tended to be interpreted by clinical teams to mean ‘We are not really sure if this bug is susceptible to this antibiotic, so probably safest not to use it’. In light of this ambiguity, the definitions of S, I and R were updated in 2019 to S, D and R.
“S” means “susceptible - standard dosing regimen”. A microorganism is categorised as susceptible when there is a high likelihood of therapeutic success using a standard dose of the antibiotic.
“D” means “susceptible,Dose Dependent ”. This means that the organism is susceptible, but that an increased Dose of antibiotic is needed. An increased dose can mean higher individual antibiotic doses and/or a reduction in the time between doses.
“R“ means “resistant”. A microorganism is categorised as "Resistant" when there is a high likelihood of therapeutic failure even if an increased dose of the antibiotic is used.
The updated term ‘D’ is meant to be more directive, and indicates that the organism is susceptible, but that a higher than standard dose of antibiotic is needed. Susceptibility tests reported as ‘D’ should be accompanied by a comment that says “D means that the organism is susceptible to this antibiotic, but that a higher Dose of antibiotic should be used. See Appendix 1 for further detail on interpretation of D, and when higher dosing of antibiotics may be indicated.
Please remember that multiple factors play a role in determining treatment outcome and therefore the results of antibiotic susceptibility testing should be seen as a guide to treatment, rather than a guarantee of treatment success.
- Urine quantification Changes
The laboratory will be reporting using SI units. This will see a change from Colony Forming Units per millilitre of urine (CFU/mL) to CFU per litre of Urine (CFU/L). It is important that the change in units and the power of the count is noted as the incorrect significance may be applied to a result
An example of a lower level of growth:
Old 10,000 CFU/mL Escherichia coli
New 107 CFU/L Escherichia coli
An example of a high level of growth:
Old 100,000 CFU/mL Klebsiella aerogenes
New 108 CFU/L Klebsiella aerogenes
An example of microscopy reports:
Old 796 bacteria per µL
New 0.796 x 109 bacteria per L
Old 1143.4 WBC count per µL
New 1.1434 x 109 WBC count per L
The laboratory will be using the same WBC and bacteria criteria in conjunction with clinical details to determine which urine specimens will also get a bacterial culture.
Appendix 1:
Antibiotic susceptibility result changes and dosing recommendations
The European Committee on Antimicrobial Susceptibility Testing (EUCAST) have updated their guidance on the reporting of antibiotics that are deemed susceptible or resistant. They will now be reported as:
- “R” = Resistant
- “S” = Susceptible at standard dose
- “D” = Dose Dependent”e. susceptible but only at a higher than standard dose. In most cases this replaces the term “I” or “intermediate” susceptibility
This is relevant for a few “bug-drug” combinations if the bacterial isolate is clinically significant and systemic treatment required (see Table 1)
Table 1. Bug-drug combinations where a susceptibility result “D” indicates a higher dose required
Bug |
Drug |
High Dose |
Pseudomonas aeruginosa |
Ciprofloxacin |
750mg BD PO
|
Staphylococcus aureus (incl MRSA)
|
Ciprofloxacin (Rarely used in primary care to treat Staphylococcal infection as many more suitable alternatives exist)
|
If being used as monotherapy, ciprofloxacin should be increased to 750mg BD po, however if being used in combination therapy standard dose 500mg BD is sufficient |
Streptococcus pneumoniae |
Levofloxacin |
500mg BD po |
Multiple factors, in addition to in vitro susceptibility play a role in determining treatment outcome, including the site and severity of the infection, the ability of the antibiotic to reach the site of infection, the antibiotic dose, duration and route of administration and the propensity of the organism to develop resistance to the antibiotic being used. EUCAST susceptibility reporting assumes that a particular dose of antibiotic is used in the treatment of infections for these reasons. Table 2 lists situations where higher doses should be considered, even when the organism is reported as “S” – susceptible at standard dose.
Table 2. Situations where EUCAST recommends higher than standard BNF doses
Bug |
Drug |
High Dose +/- indications |
Enterobacterales (e.g. E. coli, Klebsiella and Proteus species) |
Co-Amoxiclav |
625mg TDS po PLUS Amoxicillin 500mg TDS po Consider if high BMI, complicated infection or immunosuppressed (low dose 625 TDS po sufficient for uncomplicated UTI) |
Haemophilus species |
Amoxicillin Co-amoxiclav |
1g TDS po 625mg TDS po PLUS Amoxicillin 500mg TDS po
Consider if high BMI, complicated infection or immunosuppressed |
If you have queries or concerns about the safety of using a higher antibiotic dose in a particular patient, please discuss with pharmacy or microbiology colleagues.
Microbiology contact details:
- Royal London / Tower Hamlets 07785 720 703 / 07710 066 277
- Newham 07887 856 174
- Whipps Cross / Waltham Forest via WXUH switch 0208 539 5522 – bleep 2422