Can a chest x-ray be substituted for a tst?

What is TST?: If you mean tuberculin skin test - no. A chest X-ray may diagnose active tb.

Not as specific: The tst is an evaluation of past exposure with no, latent or active disease from tuberculosis. A chest x-ray is often done in followup looking for signs of active TB & that study may trigger additional studies.A random chest x-ray would not give the same information. You could have latent TB with no x-ray findings or x-ray findings from some other process.