What is the role of IgM reading in bone TB?

You are probably referring to an ELISA for IgM antibodies to tubercular antigens. TB serology has limited role in diagnosis of TB, though it does have a supportive role in diagnosing extra-pulmonary TB. A reading of 1.6 of IgM antibodies against TB bacilli is likely to be positive, but should be viewed in light of the controls during the test, and relevant clinical details.