UK Standards for Microbiology Investigations (UK SMI): scientific information

Scientific information related to UK Standards for Microbiology Investigations (UK SMI).


During testing process

Reactive: initial internal stage positive result pending confirmation.

Not reactive: initial internal stage negative result.

Equivocal: result is not clearly positive or negative. Further testing is required.

The term ‘equivocal’ may be different for various platforms for example ‘indeterminate’.

Inhibitory: the term ‘inhibitory’ may be different for various platforms for example ‘invalid’.

Reporting stage

These terms are used for final or preliminary reports.

Detected: report stage confirmed reactive result.

Not detected: report stage not reactive result.

Indeterminate: reactive result that cannot be confirmed.

Inhibitory: the term ‘inhibitory’ may be different for various platforms for example ‘invalid’.

Technical limitations

Limitations of UK SMIs

The recommendations made in UK SMIs are based on evidence (for example sensitivity and specificity) where available, expert opinion and pragmatism, with consideration also being given to available resources. Laboratories should take account of local requirements and undertake additional investigations where appropriate. Prior to use, laboratories should ensure that all commercial and in-house tests have been validated and are fit for purpose.

Specimen containers

UK SMIs use the term ‘CE marked leak proof container’ to describe containers bearing the CE marking used for the collection and transport of clinical specimens. The requirements for specimen containers are given in the EU in vitro Diagnostic Medical Devices Directive (98/79/EC Annex 1 B 2.1) which states: ‘The design must allow easy handling and, where necessary, reduce as far as possible contamination of and leakage from, the device during use and, in the case of specimen receptacles, the risk of contamination of the specimen. The manufacturing processes must be appropriate for these purposes’.

Selective media in screening procedures

Selective media which does not support the growth of all circulating strains of organisms may be recommended based on the evidence available. A balance therefore must be sought between available evidence and available resources required if more than one media plate is used.


Avidity measures antibody maturity by determining the binding strength of antibody-antigen interactions. IgG avidity tests may be used as an additional diagnostic tool especially in patients with IgM test reactivity. Avidity is initially low after primary infection and increases over time, usually about three months. High IgG avidity suggests that infection occurred over three months ago. Low or moderate IgG avidity results should not be interpreted as diagnostic of recently acquired infection, as low or moderate avidity antibodies may persist for many months following infection in some individuals. Health care providers and clinical laboratories involved in the care of pregnant women should be aware that avidity testing is an adjunct to the other tests and should be interpreted with consideration of the other serological tests and ideally earlier results.

Trouble shooting

When discrepant results are found these should be reviewed in accordance with the recommendations of the kit manufacturer. In certain instances consideration should be given as to whether they should be referred to the MHRA.

Uncertainty of Measurement

Uncertainty of measurement expresses (attempts to quantify) the doubt that inevitably exists when any measurement is made. In most circumstances, this relies on statistical data from repeated measurements that allow one to state the degree of confidence that a measured value lies within a certain range. In order to provide a measure of confidence in results produced by a laboratory, it is necessary to identify all factors which may contribute to variation in a process and assess their potential to influence uncertainty. Once identified, these factors must be reduced or controlled to an acceptable level and a value for the range of acceptable uncertainty assigned where possible.

Note: laboratories should therefore seek to explain their process of ‘consideration of uncertainty’ in terms which technical assessors will understand. It is likely that unless the uncertainty of measurement are expressed in statistical terms, appropriate ‘consideration’ will lead to a conclusion that there are too many variables in the process to express the uncertainty in a meaningful way.

Reference grading information

Modified GRADE table used by UK SMIs when assessing references

Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) is a systematic approach to assessing references. A modified GRADE method is used in UK SMIs for appraising references for inclusion. Each reference is assessed and allocated a grade for strength of recommendation (A–D) and quality of the underlying evidence (I–VIII). A summary table which defines the grade is listed below and should be used in conjunction with the reference list.

Quality/certainty of evidence Types of evidence
A Strongly recommended I Evidence from randomised controlled trials, meta analysis and systematic reviews
B Recommended but other alternatives may be acceptable II Evidence from non randomised studies
C Weakly recommended: seek alternatives III Evidence from documents describing techniques, methods or protocols
D Never recommended IV Non analytical studies, for example case reports, reviews, case series
  V Expert opinion and wide acceptance as good practice but with no study evidence
  VI Required by legislation, code of practice or national standard/guideline
  VII Letter/short communication/editorials/conference communication
  VIII Electronic citation
Published 6 December 2018