Can a cortisone injection into the carpal tunnel stop the numbness and tingling? if it does is that a good indication that you do have carpal tunnel?

Yes, not quite: Yes, it may help shrink swollen tissues such as tendons and thus reduce pressure on the median nerve which runs thought the carpal tunnel. This would stop the tingling numbness. The most reliable test for carpal tunnel syndrome is a nerve conduction study.

Carpal tunnel inject: I use xylocaine (lidocaine) 2% with my steroid. I draw the innervation of the median nerve on the patient's hand. If after injection numbness occurs in the median nerve distribution, i wait and see if the problem resolves or improves. The nc test is expensive, and i only order it if resolution does not occur!

Steroid injection: Steroid injection can temporarily relieve symptoms of numbness and tingling in the thumb, index, and long finger. The classic signs of carpal tunnel syndrome are numbness and tingling in those digits and nighties awakening with numbness/tingling. However, steroid injection is usually not an effective long term treatment strategy. Consider evaluation by a board certified hand surgeon.

Yes: It is usually temporary. However, of you do have relief this i'd also a good indication that the carpal tunnel diagnosis is correct and that you would also likely benefit from carpal tunnel release.