Table 6.

Using Test-Indication Curves

You have just seen a 48-year-old housewife who presents to the ED by private vehicle 8 hours after the onset of a severe headache which peaked within 10 minutes of onset. She is nauseated and complains of neck stiffness but her neurological exam is intact and she is able to fully flex her neck. You assign a pre-test probability of 10% for SAH, and await the final read by the neuroradiologist on duty even though the imaging study appears to be unremarkable to you and to the CT technician. The test treatment curves can be used like the Fagan nomogram to calculate the post-test probability of disease. Therefore you can estimate that the post-test probability will be around 0.8% should the study prove negative (Fig TIC panel B). You can also estimate that, if you had performed an LP rather than CT, the post-test likelihood would be around 2% if fewer than 1000 × 10^6/L RBC were present in the final tube of CSF (panel C), and similarly 2% if there were no visible xanthochromia. But you can also “chain” sequential test results, with the usual caveat that the tests may not be perfectly independent. Thus, after the neuroradiologist confirms that there are no signs of SAH on the CT, the same patient would have her probability of disease lowered from 0.8% pre-LP to about 0.1% after an LP negative for xanthochromia (panel D). Moreover, decision analysis would suggest that, for this patient, the risks of LP outweigh the potential benefits, since the pre-test probability of 0.8% is already below the range when LP is beneficial whether one uses erythrocyte count or xanthochromia, as indicated by the horizontal arrows on the test indication curves.