Estimating dengue under-reporting in Puerto Rico using a multiplier model

Abstract

Dengue is a mosquito-borne viral illness that causes a variety of health outcomes, from a mild acute febrile illness to potentially fatal severe dengue. Between 2005 and 2010, the annual number of suspected dengue cases reported to the Passive Dengue Surveillance System (PDSS) in Puerto Rico ranged from 2,346 in 2006 to 22,496 in 2010. Like other passive surveillance systems, PDSS is subject to under-reporting. To estimate the degree of under-reporting in Puerto Rico, we built separate inpatient and outpatient probability-based multiplier models, using data from two different surveillance systems-PDSS and the enhanced dengue surveillance system (EDSS). We adjusted reported cases to account for sensitivity of diagnostic tests, specimens with indeterminate results, and differences between PDSS and EDSS in numbers of reported dengue cases. In addition, for outpatients, we adjusted for the fact that less than 100% of medical providers submit diagnostic specimens from suspected cases. We estimated that a multiplication factor of between 5 (for 2010 data) to 9 (for 2006 data) must be used to correct for the under-reporting of the number of laboratory-positive dengue inpatients. Multiplication factors of between 21 (for 2010 data) to 115 (for 2008 data) must be used to correct for the under-reporting of laboratory-positive dengue outpatients. We also estimated that, after correcting for underreporting, the mean annual rate, for 2005-2010, of medically attended dengue in Puerto Rico to be between 2.1 (for dengue inpatients) to 7.8 (for dengue outpatients) per 1,000 population. These estimated rates compare to the reported rates of 0.4 (dengue outpatients) to 0.1 (dengue inpatients) per 1,000 population. The multipliers, while subject to limitations, will help public health officials correct for underreporting of dengue cases, and thus better evaluate the cost-and-benefits of possible interventions.

Fig 1. Schematic of model to estimate multipliers to correct for under-reporting of outpatient and hospitalized dengue cases.

Notes; Multiplier D (only) is used for the inpatient module. Multiplier D and Multiplier E are used for the outpatient module. See text for further details. PDSS = Passive dengue surveillance system. EDSS = Enhanced dengue surveillance system. CDC-DB = U.S. Centers for Disease Control and Prevention, Dengue Branch (stationed in San Juan, Puerto Rico).

Fig 2. Reported and estimated number of clincial dengue cases in Puerto Rico, 2005–2010 (with 95% CI).

Panel A: Reported and Estimated Dengue Inpatients; Panel B: Reported and Estimated Dengue Outpatients. Notes: MA = Medically Attended patient classification sub-model which includes all patients who either had a completed Dengue Case Information Form (DCIF), or had some indication in their medical records (such as specimens sent to a laboratory for dengue testing) as potentially having a clinical case of dengue. DO = In this patient classification sub-model, labeled “DCIF Only (DO),” we included only those patients (in or out) definitively recorded as potential dengue case on a DCIF. See text for further details. The 95% CI (confidence interval) is the range between the 2.5% and 97.5% confidence estimates.

Fig 3. Sensitivity analysis: Relative importance of individual multipliers in calculating the overall multiplier for reported dengue inpatients: Puerto Rico, 2010.

Notes: a: Graph plots relative importance of the individual multipliers used to calculate the overall multiplier. The wider the plotted range (i.e., bar), the greater the change in the overall multiplier. b: Results calculated for MA = Medically Attended patient classification sub-model which includes all patients who either had a completed Dengue Case Information Form (DCIF), or had some indication in their medical records (such as specimens sent to a laboratory for dengue testing) as potentially having a clinical case of dengue. PDSS = Passive dengue surveillance system. EDSS = Enhance dengue surveillance system. IgM = Immunoglobulin M. Test indicates presence of dengue IgM antibodies using an antibody capture enzyme-linked immunosorbent assay. PCR = Polymerase chain reaction. Test indicates evidence of dengue RNA using by a Reverse transcription polymerase chain reaction (RT-PCR) test. See main text for detailed description of individual multipliers and how they are used to calculate the overall multiplier.