The following factors can affect the results obtained from samples and should be taken into consideration when interpreting results.
 
 
Collection-to-Test Time
 
Delay between the collection of a sample and the testing of the sample can affect the results obtained, particularly if not stored at the appropriate temperature (see below). For bacteriological investigation it can result in:
 
·         An overgrowth of commensal organisms which can obscure any pathogens, or
 
·         Misrepresentation of the numbers of organisms present
 
 
For serological examinations, it may result in degradation of antibodies/antigens in the samples. This can result in false-negative or false positive reactions (due to cross-reaction).
 
 
 
Temperature
 
Storage of samples for extended periods at in-appropriate temperatures can result in:
 
·         Overgrowth of organisms (see above) if too warm
·         Degradation of antibodies, etc. if too warm in serological/viral samples.
·         Bacterial death if too cold.
 
In general, samples should be kept refrigerated (4-8oc) until they can be transported. The exception to this is blood culture samples, which should be kept at room temperature (15-25oc)
 
 
 
Sample Collection Time
 
In most cases the time at which a sample is collected will have no effect on results. In some cases, however, it is an important factor:
 
·         For mycobacterial investigation of urine, complete early morning urine are required (from 3 consecutive days) as this increases the possibility of isolating the mycobacteria.
·         For examination of urines for schistosomiasis, a urine sample should be taken in the middle of the day (after light exercise) as this is when the parasite is shed in the largest numbers.
 
 
 
Appropriate Sample
 
It is important to ensure that the correct sample is taken for the test required. An incorrect sample can affect the result obtained or can cause no result to be issued as it is not possible for that test to be performed on that sample. Examples include:
 
·         Mid-stream Urine for M/C/S) but a Terminal Urine for Schistomoniasis investigation
·         Virology swabs taken for bacterial investigation – the medium in the swab contains antibiotics to prevent bacterial growth and therefore cannot be used for this test.
 
 
 
 
Appropriate Collection Container
 
It is important to ensure that the sample is taken in the correct container for the test required. Examples include:
 
·         Blood samples for molecular (PCR) testing should be collected in an EDTA (red-topped) blood tube as the sensitivity of such tests is significantly reduced with clotted samples, and accurate viral loads cannot be obtained where appropriate.
·         Urine samples for routine culture (NOT TB) must be collect in sterile universals – any other container, including faeces pots, are not guaranteed sterile and could therefore contaminate the sample.
·         Swabs/urines for Chlamydia or dual testing MUST be taken in Abbott Collection Kits (orange-top) as the buffer present stabilises the sample to enhance the testing.
 
 
 
 
Which samples and containers are required for which tests can be found in the section entitled ‘Recommended Samples’. If you cannot find the test you require, contact the laboratory to discuss what samples are required.