The Estimated Lifetime Medical Cost of Syphilis in the United States

Abstract

Background: The purpose of this study was to estimate the cost of syphilis in the United States, in terms of the average lifetime direct medical cost per infection.

Methods: We used a decision tree model of the natural history of syphilis. The model allowed for numerous possible outcomes of infection, including treatment for syphilis at various stages, inadvertent treatment, and late syphilis outcomes in those who are alive and still infected 30 years after acquisition. Future costs were discounted at 3% annually. Model inputs, such as the cost and probability of each outcome, were based on published sources. The probabilities we applied yielded outcomes consistent with reported cases of syphilis by stage from national surveillance data and number of deaths due to late syphilis from national mortality data.

Results: The estimated, discounted lifetime cost per infection was $1190 under base case assumptions (2019 dollars). Treatment costs associated with late syphilis outcomes, such as cardiovascular syphilis, accounted for only $26 of the average lifetime cost per infection. Results were most sensitive to assumptions regarding the treatment cost per case of unknown duration or late syphilis. In the probabilistic sensitivity analyses, the 2.5th and 97.5th percentiles of the 10,000 simulations of the lifetime cost per infection were $729 and $1884, respectively.

Conclusions: Our estimate of the lifetime cost per infection is about 50% higher than in a previous study, a difference due in large part to our higher cost assumptions for benzathine penicillin G.

Figure 1.

Decision tree of natural history of syphilis used to estimate the average lifetime cost per infection Events in the decision tree are labeled by letters and numbers in brackets. We estimated the expected lifetime cost of syphilis, per infection, discounted to the time of infection (Event A). Of those reported as a syphilis case, the probabilities of treatment by stage (Event D and Event E) were consistent with the distribution of reported syphilis cases in the United States. Of those never reported as a case, we assumed three possible outcomes: treated for syphilis but not reported as a case (Event F), treated inadvertently through receipt of antibiotics for purposes other than syphilis treatment and/or death within 30 years from a cause other than syphilis (Event G),or alive and still infected 30+ years after infection (Event H). For those in Event H, we assumed one of six outcomes would occur (Events H-1 through H-6 in Panel B). The probability of Event H given Event C was estimated so that the decision tree would yield estimates consistent with current data regarding deaths due to syphilis in the United States.

Figure 1.

Decision tree of natural history of syphilis used to estimate the average lifetime cost per infection Events in the decision tree are labeled by letters and numbers in brackets. We estimated the expected lifetime cost of syphilis, per infection, discounted to the time of infection (Event A). Of those reported as a syphilis case, the probabilities of treatment by stage (Event D and Event E) were consistent with the distribution of reported syphilis cases in the United States. Of those never reported as a case, we assumed three possible outcomes: treated for syphilis but not reported as a case (Event F), treated inadvertently through receipt of antibiotics for purposes other than syphilis treatment and/or death within 30 years from a cause other than syphilis (Event G),or alive and still infected 30+ years after infection (Event H). For those in Event H, we assumed one of six outcomes would occur (Events H-1 through H-6 in Panel B). The probability of Event H given Event C was estimated so that the decision tree would yield estimates consistent with current data regarding deaths due to syphilis in the United States.

Figure 2.

Results of one-way sensitivity analysis of cost of syphilis: Estimated lifetime cost per infection when varying one parameter value at a time This diagram shows the estimated lifetime cost per infection when a single parameter value was changed from its base case value to its lower or upper bound. For example, when we varied the treatment cost per case of unknown duration or late syphilis while holding all other parameters at their base case values, the lifetime cost per infection was $932 when applying the lower bound and $1,625 when applying the upper bound (top entry of diagram). For ease of illustration, the parameter descriptions have been shortened; see Table 1 and Table 2 and manuscript text for more precise descriptions.