A conditional probability approach to surveillance system sensitivity assessment

Summary 

Objective

To determine the sexually transmitted diseases (STD) surveillance system sensitivity with a conditional probability approach at district level in Darregaz, a frontier town in the north of Iran.

Study design

A cross-sectional survey.

Methods

We used a sample survey of sexually active inhabitants for proxy measurement of the medical service utilization pattern for STD, and interviews with all practitioners to determine their knowledge of STD diagnosis and attitude towards STD reporting as proxy measures of actual STD diagnosis and reporting, respectively. Point estimates of the STD surveillance system sensitivity for each of the health service sectors were derived from multiplying the three proxy measures of sensitivity determinants, i.e., utilization, diagnosis, and reporting, as conditional probabilities. Estimates of sensitivity for all health service sectors were summed to obtain the overall sensitivity.

Results

The sensitivity of the surveillance system was 21.2% (95% confidence interval (CI) 15.5–25.3%) for detecting symptomatic STD. Of the sexually active inhabitants, 8.9% (95% CI 5.5–14.2%) did not use health services if they contracted STDs. The public health sector's contribution to overall sensitivity (59.6%) was greater than its proportion of service utilization for STD (45.3%).

Conclusions

The strengths of the conditional probability approach are feasibility of conducting necessary surveys, decomposing sensitivity into its determinants, and providing evidence for intervention at different points for planning purposes. This approach tends to overestimate the overall sensitivity.

Keywords: Epidemiological studies, Health services research, Iran, Population surveillance, Sensitivity

Abbreviations: CDC, Centers for Disease Control and Prevention, CI, confidence interval, PCA, principal components analysis, STD, sexually transmitted diseases