i am looking for private hospital to have some standard tests done in the siam/chitlom area. thanks.

the one across from Central World is the Police Hospital - so public

private one in that area - hmmmm perhaps closest is BNH in Silom? can get very pricy though. another option might be the Bangkok Christian Hospital also on Silom

yet another option would be if you head the other direction towards national stadium and keep going further - you will get to Hua Chiew hospital

lots of other private hospitals around town - and mind you I cant comment on their service etc, just mentioning some that are in the same vicinity as the one you asked about

cheers

will the wait for some basic tests be much longer in public hospital than a private?

I guess it depends on the tests. I tend to use Gov hospitals as there is less reason for them to 'find' something wrong. The tales I have heard about all those fancy hospitals people love so much..aaarrgghh!