To clarify, by "false positive" I mean showing abnormal results when it shouldn't. For example, if the cheap test is usually accurate, but when it's wrong it erroneously shows dangerously high cholesterol, then a more expensive but more accurate follow up test can compensate. But I agree that if the test has a lot of false negatives (showing normal levels when they're actually abnormal), it shouldn't be used at all.

What prompted my comment was, "While most of the results were similar for the three labs, 12.2 percent of the measurements reported by Theranos were outside the expected normal range for healthy people, compared with 7.5 percent for Quest and 8.3 percent for LabCorp."

To me that sounds like at least some of the tests may be erring towards false positives. More research needed.

For most blood assays, there is no such thing as a positive or negative. The normal range for cholesterol, for instance, has both an upper or lower limit. This leads me to believe that the Theranos tests are just less precise overall, which makes it hard to justify their value even at an 80% discount.