Testing for GBS

Detecting a GBS carrier

Screening for GBS is not routinely offered in the NHS. Laboratory testing for GBS is carried out at The Doctors Laboratory (www.tdlpathology.com) following the method described in the UK Standards for Microbiological Investigations, using enriched culture medium (ECM).

Two swabs (lower vaginal and rectal) need to be cultured, ideally in the last five weeks of pregnancy, to best predict GBS carriage around the time of delivery.

When the ECM test is properly performed within 5 weeks of giving birth, a negative result was shown to be 96% predictive of not carrying GBS at delivery (4% of women acquired carriage between the test and birth) and a positive result 87% predictive of carrying GBS at delivery (13% of women lost carriage in that time). The test can be performed earlier, but isn’t as reliable at predicting carriage status over longer periods. It can be performed later, although the chance of the baby arriving before the result increases.

Testing for GBS needs to be included in the pregnancy and birth plan and discussed with the healthcare professional including the treatment plan if GBS is detected. UK guidelines recommend that, when GBS is detected during the current pregnancy, the woman should be offered intravenous antibiotics in labour. This will stop most GBS infections from developing in newborn babies.

Antibiotics may affect the GBS test result. If you are taking antibiotics, or have recently, discuss this with your healthcare professional.

Procedure for testing

GBS carriage can come and go, though tests are very predictive for a period of 5 weeks or so. Testing is usually undertaken after 35 weeks of pregnancy. Two swabs need to be taken and sent to the designated testing laboratory the same day by Royal Mail Tracked 24 postage; delays in sending can affect the result. The swabs should be accompanied by a completed request form showing:

  • Patient’s name, address, date of birth and date sample taken. Please provide the address to where results are to be sent.
  • Mobile phone number for you to receive results by text message.
  • Name, full address, phone and email address of the healthcare professional to whom positive results will be sent.

The result of the test will be normally available within 3-5 working days of receipt by The Doctors Laboratory of the swabs. Results will be sent to the patient and if positive to the named healthcare professional indicating whether GBS has been isolated (positive) or not (not-isolated).