This is due to the fact that it takes time for anti-syphilis antibodies (such as the specific antibody FTA-ABS and the non-specific antibody VDRL) to develop in the patient, which is often referred to as the window of detection (i.e., no way to detect them). The following graph of syphilis antibody formation by Dr. Charles B Hicks shows very visually that these two antibodies are detectable in less than 80% of patients at the fourth week of infection. As the duration of infection increases, the antibody detection rate increases. By the 6th week of infection, antibodies are detectable in nearly 90% of patients. Only by the 9th week of infection are antibodies detectable in all patients. This is why syphilis is not immediately detectable by blood sampling in the early stages of infection.