Hepatitis B and transfusion testing – model answers to Feb 2017 tute questions
- What is the current percentage of the adult blood donor population who are seropositive for HBsAg in Port Moresby ?
My quick assessment was 10 positives from 107 tested = 9 %
This is in line with WHO data (see this web page ).
- Look at this test performance grid for a new test for Hepatitis B surface antigen: 1000 patient samples were tested
New test positive result
|New test negative|
|Reference test result positive||
|Reference test negative||
Calculate the sensitivity and specificity of this new test. Is it satisfactory for use as a screening test for hepatitis B infectivity?
Sensitivity – think of this of the percent of true positives (reference test result) that test positive with the new test- i.e. =260/295 – 88%
Specificity – percent of true negatives that tested negative – = 700/705 99%
Whilst this test is specific, one needs a high sensitivity more than specificity for a screening test – there would be too many false negatives and we would place people at risk from transfusion-related infection!
- Examine the Alere HbsAg kit insert and describe:
- Sensitivity and specificity of the test
- Possible causes of false negative or false positives
- The best sample type to be used in the test
a) Serum – 95% and whole blood -98 % – highly specific but not optimally sensitive – this is why rapid tests are NOT recommended for use for blood transfusion screening!
b) Whole blood with other anticoagulant ; non blood samples
c) Various specimen storage requirements – important; EDTA whole blood specified
4. Also examine the Alere HIV ½ assay sheet and the Syphilis sheets to answer the same questions
HIV: sensitivity 99.9% (serum), 92% (whole blood), specificity 98-100%
Syphilis: 100% (plasma or serum), specificity 100%
HOWEVER- note that manufacturer’s kit inserts will characteristically quote the data that puts the assay in the best light. Please read the in the field evaluation of the Alere HBsAg test that i will send to you by email (JCM 2015). It is important to listen/watch the 2016 HIV testing video presentation from RCPA which the graphic below explaining the relative sensitivity of rapid tests – they are never as sensitive as EIAs, ab-ag combo or PCR assays for HIV. Note also that HIV viral load is at its highest during seroconversion and so it is very important to for transfusion services to use an appropriate assay that goes positive as earliest as possible!
- What are the current WHO recommendations for blood bank serology testing? i.e. what viruses/bacteria to be checked; what sort of testing methods are recommended?
- The PMGH has started a Staff Health clinic and intends to ensure that all staff are vaccinated against Hepatitis B. Find out the current cost of a vaccine course in POM and the cost of the Alere HBsAg test. Factor in the % positivity expected in staff (use answer to Q1). Work out whether it is more cost –effective to screen staff for carriage first before vaccinating than just to vaccinate all without testing!
a) Universal vaccination – assume 100 staff vaccinated ; total cost = cost of vaccine course x 100
b) Screen first then vaccinate seronegative staff ; total cost = 100 x cost of HBsAg screen + vaccine cost x 91
Which comes in at a lower cost?
The other advantage of screening is that those staff who are positive need to know this:
- They may warrant treatment
- If they are a surgeon or other hcw involved with ‘exposure prone’ work then most countries would say they should not be doing that work unless there viral load is kept low with treatment (< 1000 copies per mL)