Coordinates of the Curve

This table reports the sensitivity and 1-specificity for every possible cutoff for positive classification. The sensitivity is the proportion of HIV-positive samples with assay results greater than the cutoff. 1-specificity is the proportion of HIV-negative samples with assay results greater than the cutoff.

Cutoff 0 is equivalent to assuming that everyone is HIV-positive. Cutoff 9 is equivalent to assuming that everyone is HIV-negative. Both extremes are unsatisfactory, and the challenge is to select a cutoff that properly balances the needs of sensitivity and specificity.

For example, consider cutoff 5.5. Using this criterion, assay results of 6, 7, or 8 are classified as positive, which leads to a sensitivity of 0.978 and 1-specificity of 0.015. Thus, approximately 97.8% of all HIV-positive samples would be correctly identified as such, and 1.5% of all HIV-negative samples would be incorrectly identified as positive.

If 2.5 is used as the cutoff, 99.5% of all HIV-positive samples would be correctly identified as such, and 4.0% of all HIV-negative samples would be incorrectly identified as positive.

Your choice of cutoff will be mandated by the need to closely match the sensitivity and specificity of traditional tests. Please note that the values in this table are at best guidelines for which cutoffs you should consider. This table does not contain error estimates, so there is no guarantee of the accuracy of the sensitivity or specificity for a given cutoff in the table.

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