Potential Biases When Measuring Repeat Chlamydia Trachomatis Infection Rates to Assess the Real-World Effectiveness of Expedited Partner Therapy, New York City, 2011-2013

Background:  Efficacy of expedited partner therapy (EPT) for preventing repeat Chlamydia trachomatis (Ct) infection was established by randomized trials. EPT has been implemented in many jurisdictions, however, it is not clear how best to assess its real-world effectiveness.

Methods:  In New York City’s (NYC) public STD clinic system, heterosexual patients with laboratory-confirmed Ct, without concurrent gonorrhea or syphilis are eligible for EPT. For EPT-eligible patients who did, and did not receive EPT during 2011-2013, we compared: follow-up Ct testing rates 3-6 months after Ct treatment; repeat laboratory-confirmed Ct infection; select characteristics.

Results:  There were 8258 EPT-eligible patients; the majority (60%, 4975/8258) did not receive EPT, mostly (84%, 4187/4975) because they were presumptively treated.  Among 3283 patients offered EPT, 58% (1909/3283) received EPT; the main reason patients declined EPT was partner(s) were already treated (31%; 432/1374). Retesting rates were low after treatment, especially for those who did not receive EPT (11% (691/6349) versus 17% (333/1909) of those who received EPT, p<0.0001). Repeat infection rates were higher among patients who did not receive EPT (9%, (59/691) versus 3% (6/333) of those who received EPT, p<.0001). Patients who did not receive EPT were significantly more likely to be: male (60% (3783/6349) versus 28% (527/1909), p<0.0001); presumptively treated (70% (4446/6349) versus 17% (324/1909), p<0.0001); report  >2 sex partners (38% (2415/6349) versus 29% (540/1909), p<0.0001); and a contact to Ct (41% (2586/6349) versus 5% (101/1909), p<0.0001). Restricting analysis to persons offered EPT gave different results, but groups were still dissimilar, suggesting selection bias.

Conclusions:  Repeat infection rates were lower among persons who received EPT.  However, patients who received EPT were substantially different from those who did not, and both groups had very low rates of follow-up testing. Misclassification and selection bias could easily mask the actual effects of EPT in observational studies of non-randomized populations.