Spurious results

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

However, the pre-examination / pre-analytical phase plays an important part in the cause of spurious results produced by the laboratory. This phase is largely under the control of the user/requester.

Below are the 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 us if in any doubt as to the validity of results.

Causes and effects

Delay in processing

Cause: overnight storage, delay in transport, delayed analysis within laboratory

Effect: overgrowth of contaminating organisms, changes in pH in samples may lead to death of target organisms

Temperature

Cause: storage of blood cultures in fridge

Effect: delay in detection of organisms

Haemolysis

Cause: poor collection technique, difficult to bleed, frozen storage, delayed transit/analysis

Effect: a high degree of haemolysis can cause inaccuracy in serological tests such as HIV and HCV

Lipaemia (turbid sample)

Cause: sample taken shortly after fatty meal

Effect: erroneous results

Multiple freeze-thaw cycles

Cause: request for retrospective testing

Effect: for serum/plasma samples for serological testing >3 freeze-thaw cycles may affect results

Incorrect sample site

Cause: sample collected from sub-optimal site

Effect: decreased sensitivity for recovery of pathogens

Lower than expected concentration measured

Cause: use of liquid anticoagulants

Effect: dilutes sample - more pronounced the smaller the sample volume

Incorrect sample size

Cause: sample volume too low for testing

Effect: 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

Cause: sample integrity not protected on transit or during testing

Effect: decreased sensitivity for recovery of pathogens

Sample centrifuged before clot formation

Cause: patient receiving anticoagulant or thrombolytic therapy

Effect: erroneous results

Cross-reactivity

Cause: interfering exogenous or endogenous substances

Effect: false positive or negative reactions depending on cross-reaction

No growth

Cause: antibiotics given prior to sample being taken

Effect: antibiotics given prior to sample collection can deplete live organisms leading to failure in culture

Mixed growth

Cause: see 'delay in processing', also catheter samples and samples from pregnant women are more prone to contamination

Effect: contamination of samples with skin or faecal flora when samples are delayed in transport, or it is difficult to produce a 'clean' sample

Sample mismatch

Cause: incorrect labelling by sender

Effect: sample not processed

Tested for incorrect test

Cause: unclear manual request, incorrect test requested electronically

Effect: sample testing delayed or not tested at all

Contamination at source or during transportation

Pay close attention to optimum sampling techniques, including aseptic technique. Ensure samples are sealed appropriately to avoid subsequent contamination.

Inappropriate timing of sample

Whenever possible, send samples to Microbiology prior to initiation or change of antibiotic therapy. When this is not possible, prioritise sample collection as soon as possible after the first dose of antibiotics.

Septic shock is a clinical emergency and urgent antibiotic treatment should always be initiated; do not delay if it is difficult or unsafe to obtain cultures.