Reasons for discordant results

Answer

• Discordant results are when you get a test result through one testing service and then get a different result with another service. This can happen because these services using different types of tests, or different testing platforms.
• Sometimes people who become infected with an STI may naturally clear the infection or may receive treatment for another healthcare conditions (e.g. dental infection) which inadvertently treats the STI. Testing either side of this natural clearance or treatment clearance can generate different results.
• If you have been recently infected with an STI, it can take a period of time before your test becomes positive. These are called window periods. Window periods differ in length according to the STI in question but it can result in an initial test being negative and a second test a few days later becoming positive.
• The window period for gonorrhoea and chlamydia is approximately two weeks. The time it takes until the test becomes positive after infection can also vary according to what type of test is used. For example, some tests that are performed in a sexual health clinic and sent to a laboratory may turn positive a few days before the finger prick HIV tests do because they have a different window period before detecting infection.
• If you test very soon after condomless sex, a positive test result could theoretically be detecting infection in your partners' body fluids. If the STI is not transmitted from your partner to you, then your test result can become negative later.
• If someone does not follow the instructions when taking their samples, this can produce a negative result in an infected person. For example, if you pass urine 10 minutes before collecting your urine sample. Conversely if you drop or touch the swab tip whilst self- sampling, this can contaminate your sample leading to a positive result.
• Test me, as well as other remote testing services, requires you to post your samples back to a laboratory. This can increase the number of blood samples that haemolyse (clot) and, although we quality check all samples when they are received, sometimes this can cause an incorrect result. Haemolysis does affect samples taken in clinic too but to a slightly lesser degree.
• Test me laboratory processes are extremely stringent and overseen by experienced quality, medical and laboratory management teams in accordance with international accreditation. However, sometimes there are problems in the laboratory, and we may undertake an investigation for certain discordant cases.
• We are often asked to explain why one partner may test positive for an infection whilst the other partner tests negative. Some of the circumstances above can explain these differences. It is important to check there are no other partners where an STI could have come from. It is also important to ensure the tests you perform are nationally recommended and from an accredited organisation. Again, a discussion with the clinical team can often help identify why there is a difference between you and your partner's results and whether you should consider having treatment.