Assessing sensitivity of case-finding using capturerecapture techniques

By Johannah Wegerdt, Monica Zanchettin and Mark Myatt

Marking the midpoint during MUAC measurement

Monica Zanchettin is a coverage surveyor for Valid International. She has worked for the International Organisation for Migration in Ethiopia and Hungary and has completed MSc studies in Forced Migration and an MPhil in Advanced International Studies.

Johannah Wegerdt currently works as an epidemiologist in the UK. She previously worked for Valid International, conducting coverage surveys in Malawi, Ethiopia, South Sudan, Darfur and Zambia between 2003 and 2005, and worked as an epidemiologist for MSF in Uzbekistan (2000 to 2001).

Mark Myatt is a consultant epidemiologist and senior research fellow at the Division of Opthalmology, Institute of Opthalmology, University College London. His areas of expertise include infectious disease, nutrition and survey design.

Recording the MUAC value

Capture-recapture studies are used to estimate the size of a population when a census may be infeasible or impossible to conduct. The basic idea of capturerecapture studies is to sample and identify individuals, or cases, from a population and then resample the population to see what fraction of individuals, or cases, in the second sample were identified in the first sample (i.e. the fraction of individuals, or cases, that were found in both samples).

In this article we will explore the use of capturerecapture techniques to estimate the sensitivity (also called exhaustivity) of two case-finding methods. The sensitivity of a case-finding method is a measure of how well the method performs at finding cases. This is useful for:

1. Selecting a rapid case-finding method for use by outreach workers.
2. Selecting a rapid case-finding method for use in coverage surveys using the new centric systematic area sampling (CSAS) method.

In both cases we would want a rapid case-finding method that finds all or nearly all cases (i.e. a rapid case-finding method with a high sensitivity).

Figure 1 shows a capture-recapture study in diagram form. In this diagram:

N = Total number of cases in the study population
M = Number of cases found in the first sample
C = Number of cases found in the second sample
R = Number of cases found in both samples

For this application one sample will usually be collected using a census (or census-like) sampling method such as central-location screening or house-to-house screening and the other sample will be collected using a rapid case-finding method.

Figure 2 presents a capture-recapture study with four sets of cases labelled a, b, c, and x. In this diagram:

N = a + b + c + x
M = a + b
C = a + c
R = a

Where x is unknown. This information can be presented as a two-by-two table (Figure 3).