Table 1.
Model variables and sources.
Figure 1.
Simplified overview of screening and treatment component of the decision analytic model.
A decision is made to screen or not screen HIV-positive MSM at risk of syphilis acquisition. If screening is performed, men can be correctly or incorrectly classified, resulting in appropriate or inappropriate use of resources, with associated costs and health consequences. Syphilis is treated according to the current Canadian guidelines. Chance nodes indicate points at which probabilities (described in Table 1) are applied. Individuals progress to the appropriate health state (outlined in Figure 2) following progression through the screening and treatment decision tree. Note that men who seek treatment for symptomatic syphilis infection follow the same set of decisions.
Figure 2.
The model has eleven health states, with allowed transitions between states indicated by arrows. Men have a chance of remaining uninfected or acquiring syphilis and progressing through the disease states. Syphilis infection is characterized by four stages: primary, secondary, early latent, and late latent, which may develop into tertiary syphilis. Men only exit the tertiary syphilis state via death, even if they receive treatment. Men may develop neurosyphilis at any stage of their syphilis infection. Men with early syphilis may receive treatment and recover without disability or have lifetime disability (indicated by entry into the tertiary syphilis state). All men with late neurosyphilis are assumed to have lifetime disability and enter the tertiary syphilis state. Men may transition to the death state from any model state. Treatment results in men returning to the ‘previously infected and treated' state. Movement through the model health states depends on transition probabilities identified from the literature. Disutilities are associated with the primary, secondary, tertiary, and neurosyphilis states and long-term healthcare costs are associated with tertiary and neurosyphilis.
Figure 3.
Model validation and projections.
Model estimated diagnosis of early neurosyphilis, infectious (primary, secondary, and early latent) syphilis, and false positive cases. Reported values represent the average rates in the modeled cohort over a 20-year period for the different strategies evaluated. Neurosyphilis infections are plotted x100 for comparability. Usual care – annual represents model projections based on current estimates of screening coverage and frequency among HIV-infected MSM under medical care. Toronto HIV-infected men represents estimated rates of diagnosed early neurosyphilis among HIV infected men living in Toronto (average for the years 2008–2012, error bars represent 95% confidence intervals). Usual care, usual 6 months, and usual 3 months refer to screening 57% of the population every 12, 6, or 3 months, respectively. Higher coverage annual, 6, months and 3 months refer to screening 100% of the population every 12, 6, or 3 months, respectively.
Table 2.
Discounted health and economic outcomes associated with different syphilis screening strategies.
Figure 4.
Strategy acceptability for different willingness-to-pay thresholds.
The frequency with which each strategy was optimal at willingness-to-pay thresholds of 0, $50,000, or $150,000 per QALY is shown for 1000 probabilistic trials with 1000 individuals assigned to each strategy within each trial. Usual care, usual 6 months, and usual 3 months refer to screening 57% of the population every 12, 6, or 3 months, respectively. Higher coverage annual, 6, months and 3 months refer to screening 100% of the population every 12, 6, or 3 months, respectively.
Table 3.
Preferred syphilis screening strategies under alternate model assumptions and for different willingness-to-pay thresholds.
Table 4.
Undiscounted health and economic outcomes associated with different syphilis screening strategies.