How long after the last dose of isoniazid will the test remain positive?
Initial tests (Annals of Clinical Biochemistry 2004; 41: 411-413) and an ongoing evaluation shows the test is positive for 24-30 hours after the last dose. If the drug ingestion was longer than this but less than 48 hours the resultant test colour will be green. Any longer than 48 hours the test colour will remain yellow. However, this will be affected by the acetylator status of the patient.
If I think that a colour is developing should I leave it longer than 5 minutes?
No – leaving the test longer could give spurious results. If isoniazid or its metabolites are present in the urine it will turn dark blue very quickly.
Why do people not take their anti-tuberculous therapy?
There are many reasons, but forgetting to take the tablets is a major cause, especially towards the end of the therapy programme. Also, towards the end the patient will feel they have cured the TB yet they know the tablets are causing side-effects, hence they stop taking the tablets.
When using the test what is the shortest and longest time I can leave before reading the result?
Normally, a positive sample will change colour within seconds, but it is recommended that the full 5 minutes be left to get a definitive test result.
How do I dispose of the test after use?
It is recommended to place the used test in a 'burn' or 'sharps' bin for disposal.
Can any other substance interfere with the test?
Results suggest there is no other drug or substance that will give the dark blue/purple colour of a positive IsoScreen test.
What is the sensitivity and specificity?
Results suggest that the sensitivity is 97% and specificity is 98%.
Can I use the urine sample as soon as it has been voided?
The test should ideally be used when the urine is at room temperature, but the qualitative nature of the test means a correct result will be obtained with the sample at body temperature.
Will the tests pick up other TB drugs?
Test results show this not to be the case.
What is the test result if I am not sure whether the tests has changed colour?
The test has to be viewed under good lighting conditions, and then if there is any doubt if there has been a colour change then the result is negative. There has to be a visible blue/violet colour change for the result to be positive.
How should I store the tests?
The tests should be stored at room temperature.
What is the shelf-life of the test?
The tests have a shelf-life of at least 12-months after manufacture. The expiry date is printed on the pouch of box.