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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