Studies before 1990 indicated that if patients with their first syphilitic infection in the primary or secondary stage were given adequate treatment, an elevated VDRL titer would decline (on the average) 2 dilutions (fourfold) by 3 months and 3 dilutions by 6 months. The rate of decline was said to be faster when the pretreatment VDRL titer was higher and more slowly when the VDRL titer was lower or when the infection was long-standing or was a reinfection. A large Canadian study published in 1991 confirmed some of these observations and disagreed with others. The Canadian study found that the first infection and the earlier stages of syphilis were more likely to respond serologically to treatment and responded faster, in agreement with previous studies. However, the Canadian study, using the RPR and current treatment recommendations, noted a slower rate of RPR decline, with a 2-tube dilution decrease (on the average) not attained until 6 months and a 3-tube decrease not achieved until 12 months. Although the rate of decrease was similar at all dilutions, the time needed to decrease to nonreactive was (as expected) much earlier with lower pretherapy titers than with high titers. In first-infection primary syphilis, if the initial titer was 1:8, the RPR (on the average) became nonreactive in 26% of patients by 3 months, in 54% by 6 months, in 70% at 1 year, and in 81% by 3 years. If the initial titer was 1:256, no patients were nonreactive at 3 months, only 6% at 1 year, and 31% at 3 years. In the early latent stage with a pretherapy titer of 1:8, 7% were nonreactive at 3 months, 31% at 1 year, and 66% at 3 years.

In the Canadian study the FTA-ABS became nonreactive after therapy in 11% of first-infection primary stage patients at 1 year and 24% at 3 years. The FTA-ABS or the MHA-TP seldom became nonreactive when therapy began in the secondary stage and in no patients after the secondary stage.

One study reported that HIV infection in its late stage may result in loss of FTA-ABS and MHATP reactivity in 17% of cases. In one study, BFP results were obtained using the RPR method in 4% of HIV-positive patients (all in males) versus 0.8% incidence in non-HIV patients. Another publication noted that whereas HIV infection may delay VDRL seropositivity, eventually the test does become reactive. Another noted that the term “dils” (dilutions) in a VDRL or RPR report refers to the titer by using the total volume of the diluted specimen (i.e., a titer of 1:8 may be reported as 8 dils).