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Below are the specific answers for your questions.
CMIA is based on chemiluminescence principle and it is performed on an automated analyzer. It is the most effective technique nowadays to rule out HIV. CMIA can almost 100 % rule out HIV after 90 days of exposure. Usually, in most of the patients, CMIA can be effective after six weeks of exposure. So after 175 days of exposure, you need not to worry about HIV.
The index value is cut off value based on which the result is given. The chance of error is very less. After testing for two times the chance of error is almost nil. After three years of exposure there is no medical issue, so the chances of HIV is very less.