Hair follicle testing provides the longest detection window of any drug test — approximately 90 days. A small sample of hair (about 100 strands, 1.5 inches long) is typically collected from the crown of the head. The test detects drug metabolites that have been incorporated into the hair shaft through the bloodstream. It is particularly effective at detecting patterns of repeated drug use but may not detect very recent use (within the last 7-10 days). Currently approved for some DOT testing situations and widely used in non-DOT employer programs.