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.
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.