Normally each test piece (usually an encased sphere) is supplied with a certificate of conformance (or equivalent title) at time of purchase. Also this document is usually requested to see by an auditor.
If you don't have one, may well be cheaper to acquire a new test piece(s).
Regarding the limit, many countries have their own legislatory standard(s), albeit often borrowed from elsewhere. Malaysia i I don't know. ? Legislative limits may also depend on the product / intended consumer, eg similar to USFDA.
The MD should also come with a manual containing (minimum) detection specification for a given standard test piece. If it complies with yr test, this presumably implies that MD working effectively. Some people regardless have their MD "officially" serviced / calibrated at least annually so as to generate documentation. May also depend on yr audited standard. Previous frequency is unrelated to the frequency of usage of test pieces when MD is in routine inline operation.
Rgds / Charles.C