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Using microbiology services

The microbiology laboratory can help to; provide or confirm a diagnosis, suggest appropriate antibiotics and monitor response to treatment.

Failure to investigate may lead to:

  • an increased use of antibiotics causing possible harm to patients.
  • an increasing reliance on expensive new broad-spectrum agents
  • increasing antibiotic resistance in the community (and concomitant lack of knowledge of this)
  • difficulty in establishing a diagnosis when patients have failed to respond to treatment

Doctors requesting investigations should bear in mind Asher’s catechism,
(British Medical Journal 1954; ii: 460):

  1.  Why do I request this test?
  2.  What will I look for in the result?
  3.  If I find what I’m looking for will it affect my diagnosis?
  4.  How will this investigation affect my management of this patient?
  5.  Will this investigation ultimately benefit this patient? 

This will ensure that laboratory services are used in a cost effective manner.

Sample acceptance and rejection – please refer to the pathology wide policy viewable under the general pathology information page.

Routine investigations

Specimens for routine investigations should be collected as early in the day as possible to ensure that they arrive in the laboratory during normal working hours.

Any specimens sent from patients suspected or diagnosed of blood borne viral infections, must be identified by using the yellow “danger of infection” sticker both on the specimen, and on the request form.

Bacteriology samples – reports

Significant bacterial isolates will be reported as present in either a ‘heavy growth’, a ‘moderate growth’ or a ‘scanty growth’ or in some cases where it is the only isolate and is an established pathogen simply being ‘isolated’.

The amount of growth corresponds to the amount of bacterial growth obtained from the sample on the culture plates. Heavy growth indicates that the organism was present in all areas of the culture plates. Moderate growth indicates half the plate was covered. Scanty growth indicates that the organism was only present in the initial sample inoculum.

The amount of growth does not necessarily correspond to the degree of infection in the patient. As this can depend on a number of factors including how well the swab was taken, prior antibiotic use, wound dressings etc. But it is given as a guide to assist clinicians in deciding the significance of isolates, when they are present in mixtures.

Antibiotic susceptibility results are reported as:

  • S -sensitive
  • R -resistant
  • I- intermediate

Interpretation is made using the EUCAST guidelines.

Susceptible (or sensitive): a bacterial strain is said to be susceptible to a given antibiotic, when it is inhibited in vitro by a concentration of drug that is associated with a high likelihood of therapeutic success.

Intermediate: the sensitivity of a bacterial strain to a given antibiotic is said to be intermediate when it is inhibited in vitro by a concentration of the drug that is associated with an uncertain therapeutic effect.

Resistant: a bacterial strain is said to be resistant to a given antibiotic when it is inhibited in vitro by a concentration of the drug that is associated with a high likelihood of therapeutic failure.

The classification ‘intermediate’ means that the organism may well be eliminated in body parts that are easily accessible by the drug (e.g. urinary tract), while the same may not be adequately effective against the same organisms, if it is located at other site (e.g. meningitis).

Sensitivity testing is used to determine the right antibiotic treatment for an infection and to monitor changes in bacterial resistance to antibiotics.

If any further advice is required as to the significance of any isolate or the meaning of any antimicrobial susceptibility tests, please contact the Consultant Microbiologists.

Requests for extra tests

These should be telephoned to the laboratory as soon as possible. And a request form should be sent within 4 hours, to confirm the request for the extra test saying “sample already in lab”.

Please note that due to the potential for contamination from the blood science analysers, tests cannot be added onto samples already tested in blood sciences. It is necessary to send a separate sample and form, to microbiology. If the patient is difficult to bleed or there is some other reason that a blood sciences blood will need to be used please contact the laboratory to discuss.

Samples submitted for bacterial culture are only suitable for re-testing for 48 hours, so if extra tests are needed after this time a fresh sample should be sent.

Blood specimens for serology are stored for 2 years, however extra tests should be requested in relation to the current clinical episode. If unsure please seek advice from the Consultant Microbiologist.

Any other specimens - please discuss with laboratory staff or a Consultant Microbiologist.

Uncertainty of Microbiology results

The laboratory has determined uncertainty of measurement for all assays where a numerical value is reported or used to determine the result. These values are obtainable by contacting the laboratory.

For other tests where a numerical value is not part of the determination of the result, the laboratory has undertaken a quality risk assessment. The risk assessment ensures all the factors that are known to affect the quality of the result have been taken into consideration, and actions are taken to minimise the risks.

The consultant microbiologists will be happy to discuss any results which you feel may have been influenced by uncertainty.