Blood banks are the biggest customers, using 41 million tests annually, according to Frost & Sullivan, that cost a minimum of $1.29 per ELISA. Another 1 million or so people a year get tested voluntarily. Positive samples go through additional confirmatory ELISAs, as well as a Western blot test, which detects HIV proteins directly. In May, the FDA gave Johnson & Johnson and Chiron the green light to begin marketing Confide, the first kit that allows people to take a small blood sample at home and then send it in for an analysis. The test retails for about $40.