Minimal retesting intervals (MRI) are defined as the minimum time before a microbiology test should be repeated and are based on the properties of the test and the clinical situation in which it is used.
There are national guidelines published by the Royal College of Pathologists which indicate MRIs for some sample types. These guidelines can be found on the Royal College of Pathologists website.
Duplicate samples received within the MRI are not processed by the laboratory and are rejected with a report explaining why the sample has been rejected. Certain high risk groups are excluded from this process, please see table below for details. If the sample type you are sending does not appear listed in the table below, then there are no minimum retesting intervals and all samples received will be processed.
Sample types and tests with no exceptions
|Specimen type/test||Minimum retesting interval|
|Cough swab for bacterial culture||7 days|
|Ear swab for bacterial culture (on the same ear)||7 days|
|Faeces for C. difficile testing (negative on 3 samples)||28 days|
|Faeces for C. difficile testing (positive samples)||28 days|
|Faeces for viral PCR||7 days|
|Faeces for microscopy of ova, cysts and parasites||1 day|
|Mouth swab for bacterial culture||7 days|
|Nasopharyngeal aspirate for bacterial culture||7 days|
|Nose swab for bacterial culture||7 days|
|Throat swabs S. pyogenes (negative result)||3 days|
|Throat swabs S. pyogenes (positive result)||7 days|
|Midstream and catheter urine samples for bacterial culture||3 days|
Sample types and tests with exceptions
|Sample type/test||Minimum retesting interval||Exceptions to the MRI|
|CPE screening swabs||7 days||Contact of a CPE case in the hospital.|
Sample requested by infection control as part of an outbreak/enhanced surveillance.
|Bronchial lavage samples (BAL) for bacterial culture||3 days||BAL samples sent for mycobacterial culture.|
ITU or HDU.
|Eye swab for bacterial culture (on the same eye)||7 days||Neonates|
|Faeces for bacterial PCR||7 days||Samples sent for clearance testing as per public health guidance|
|MRSA screen from the same site||7 days||If required as a clearance sample for hospital transfer|
|Seminal fluid for bacterial culture||28 days||Prostatic secretions|
|Skin scrapings, nail and hair for mycology culture||3 months||Skin biopsy samples|
|Sputum for bacterial culture||3 days||Excludes sputum sent for mycobacterial culture|
|Wound swabs for bacterial culture from the same site||7 days||Diabetic foot|
If there is a clear clinical indication for repeating a test within the minimum retesting interval for the sample type, then a formal request should be made to [email protected]. Please send this a soon as possible and provide the following information in the request:
- Patient name and date of birth or NHS number
- Sample type
- Laboratory number for the rejected sample
- Specific clinical information to justify why the sample should be tested within the minimum retesting interval