Would negative result rule out syphilis at 28 days after exposure?

Not with certainty: Primary syphilis causes a PAINLESS ulcer on the genitals. VDRL test becomes positive 2 to 5 weeks after infection. A negative test at six weeks definitely excludes the diagnosis. Note false positives may occur so in all you should see a doctor with expertise in this field. There are experimental tests at Medical Centers, such as PCR which may give a positive result earlier.

Depends on tests: Common screening tests such as RPR and VDRL may turn out negative because only about 20% of individuals exposed test positive at 4 weeks after exposure. Treponema specific test such as TP-PA (Treponema Pallidum Particle Agglutination) tests positive earlier. Negative RPR and TP-PA test at 6 weeks will be more assuring.