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.
