Oxford University Hospitals NHS Foundation Trust
Microbiology

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Causes of spurious results

The laboratory has systems in place to reduce errors to a minimum during the analytical phase of the sample.

However, as you can see from the table below the pre-examination / pre-analytical phase also plays a part in the cause of spurious results produced by the laboratory. This phase is largely under the control of the user/requester.

The table below shows common causes of spurious results and the effects seen.

Results may not be reported if it is clear that the result is invalid.

This list is not exhaustive; please contact the laboratory if in any doubt as to the validity of results.

Problem Cause(s) Effects
Delay in processing Overnight storage, delay in transport, delayed analysis within laboratory Overgrowth of contaminating organisms, changes in pH in samples may lead to death of target organisms
Temperature Storage of blood cultures in fridge Delay in detection of organisms
  Hot weather As for 'Delay in processing'
  Hot stool not arriving in laboratory within 30 minutes No parasites seen
Haemolysis Poor collection technique, difficult to bleed, frozen storage, delayed transit/analysis A high degree of haemolysis can incur inaccuracy in serological tests such as HIV and HCV
Lipaemia (turbid sample) Sample taken shortly after fatty meal Erroneous results
Multiple freeze-thaw cycles Request for retrospective testing For serum/plasma samples for serological testing >3 freeze/thaw cycles may affect results
Incorrect sample site Sample collected from sub-optimal site Decreased sensitivity for recovery of pathogens
Lower than expected concentration measured Use of liquid anticoagulants Dilutes sample - more pronounced the smaller the sample volume
Incorrect sample size Sample volume too low for testing Not all tests requested will be performed, sub-optimal volume may be used for testing leading to false negative results (laboratory will always add comment to a report where low sample volume has been used or the sample has been diluted for testing)
Incorrect sample tube Sample integrity not protected on transit or during testing Decreased sensitivity for recovery of pathogens
Sample centrifuged before clot formation Patient receiving anticoagulant or thrombolytic therapy Erroneous results
Cross-reactivity Interfering exogenous or endogenous substances False positive or negative reactions depending on cross-reaction
No growth Antibiotics given prior to sample being taken Antibiotics given prior to sample collection can deplete live organisms leading to failure in culture
Mixed growth See 'delay in processing', also catheter samples and samples from pregnant women are more prone to contamination Contamination of samples with skin or faecal flora when samples are delayed in transport or it is difficult to produce a 'clean' sample
Sample mis-match Incorrect labelling by sender Sample not processed
Tested for incorrect test Unclear manual request, incorrect test requested electronically Sample testing delayed or not tested at all

Also refer to our test pages for more details on the type of samples accepted for each test.