Define a set of algorithms
The algorithms proposed are based on the screening tools chosen in the previous pages. Users can tick to select the algorithms to include in the simulation. Algorithms conform to those recommended by WHO. A graphic depiction of the algorithms can be seen by clicking on the right screen. Please refer to Chapter 3 and the annexes of the WHO TB screening handbook for more information.
Algorithms
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Algorithm images
A single screening algorithm uses a single screening test. Those with positive or abnormal result go on to diagnostic evaluation including bacteriologic testing for TB disease. Those with negative or normal screen are considered free of TB disease, can be referred for TPT if eligible.
This is the most simple, straight-forward algorithm to implement, and it is sufficient if using screening and diagnostic tests of sufficient accuracy.
A parallel screening algorithm uses two screening tests conducted simultaneously. Those with a positive or abnormal result on either screening test go on to diagnostic evaluation including bacteriologic testing for TB disease. Those with negative or normal result on both tests are considered free of TB disease, can be referred for TPT if eligible.
This is a more sensitive algorithm than a single screening test approach, but less specific - meaning it will detect more prevalent TB but will also result in diagnostic evaluation for more people who don’t have TB.
A sequential positive screening algorithm uses two screening tests conducted sequentially. Those with positive or abnormal results on the first screening test go on to a second screening test. Those with a positive or abnormal result on the second screening test go on to diagnostic evaluation. Those with negative or normal results on the first test and the second test are considered free of TB disease, can be referred for TPT if eligible.
This approach is more specific than a single or parallel screening approach but less sensitive – meaning it is more efficient at selecting people who have the highest risk of TB for diagnostic testing, but it won’t detect all prevalent TB cases.
A sequential negative screening algorithm uses two screening tests conducted sequentially. Those with positive or abnormal result on the first screening test go on to diagnostic evaluation. Those with a negative or normal result on the first test go on to a second screening test. Those with a positive or abnormal result on the second screening test go on to diagnostic evaluation. Those with negative or normal results on the first test and the second test are considered free of TB disease, can be referred for TPT if eligible.
This approach achieves the same high sensitivity of a parallel screening approach but saves resources – the more expensive screening test (e.g. CXR) is only conducted among those who do not already screen positive with the less expensive screening test (e.g. symptom screening).
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