Blood test identifies Alzheimer's with 86% accuracy and differentiates forms

I never know how to interpret accuracy ratings for tests. If I told everyone I ran into on the street that they did not have Lou Gehrig's disease, regardless of any testing, I would still be 99.993% accurate. Can someone shed some insight on what 86% means?

Info on the 86%<p>&gt; The researchers say that using the test, they were able to identify Alzheimer&#x27;s patients with up to 86% sensitivity and specificity. (Sensitivity refers true positives identified by the test, while specificity refers to true negatives.)