5th International AIDS Society Conference on HIV Pathogenesis and Treatment Cape Town - July 19 - 22, 2009

A NEW PARADIGM FOR INCIDENCE ESTIMATION FROM CROSS-SECTIONAL DATA

IAS Conf HIV Pathog Treat 2009 Jul 19-22;5th: Abstract No. MOPDB105

T.A. McWalter¹, A. Welte^1,2
1University of the Witwatersrand, School of Computational and Applied Mathematics, Johannesburg, South Africa, ²South African Centre for Epidemiological Modelling and Analysis (SACEMA), Stellenbosch, South Africa

BACKGROUND: Incidence measurement using cross-sectional surveys is an attractive approach because it is cheaper, quicker and easier to implement than prospective follow-up. However, there is currently controversy over which assays constitute robust tests for recent infection (BED, Avidity, etc.) and which estimation formulae to use (McDougal estimator, Hargrove estimator, etc.). A key challenge is to account consistently for imperfect long-term specificity (i.e. a subpopulation of assay non-progressors).

METHODS: We provide a formally rigorous analysis to derive a new incidence estimator, which, apart from sample counts, depends on two assay-specific parameters (window period and long-term specificity). We also derive expressions for confidence intervals. The estimator is tested against a model epidemic and applied to field data where its performance is compared to incidence from prospective follow-up. We also compare the new estimator with other published estimators for a general steady state epidemic.

RESULTS: The key findings for the new estimator are:

1. It is applicable under more relaxed assumptions than any of the previous estimators.
2. Robust results are achieved when applied to a model epidemic which includes significant transients.
3. Calculated confidence intervals are accurate under model epidemic conditions.
4. When compared to the other estimators, it is the least biased.
5. When applied, under consistent calibration, to field data it performs well compared to the incidence found by prospective follow-up.

CONCLUSION: This work illuminates some of the issues that have lead to the controversy surrounding incidence measurement from cross-sectional surveys. In particular, it presents a consistent estimator and highlights the crucial role of calibration of that estimator.

2009-07-22
MOPDB105
Poster Discussion MOPDB1 - Update on Diagnosing and Monitoring

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