Is 5 days post sexual encounter too soon for testing for syphilis and hiv?

Yes, 5 days is much too soon. It takes several weeks for the blood tests to become positive. However, in a very high risk situation for HIV (for example, unprotected sex with a known infected partner), preliminary testing can start with RNA PCR testing 10-14 days after exposure. But these tests are expensive and not recommended except in especially high risk situations.

False negative tests occur when you test too early. It takes various time periods for the body to respond to different std's. Syphilis can show symptoms from 10-90 days with an average time of 21 days. Test by 90 days anyway. Watch for appearance of a sore. Test for HIV between 30-90 days. These are guidelines You have a high chance of infections if you don't know the sexual health of your partner