At first glance it it looks as if those two tests you mentioned may have been run on different equipment. To get an exact reading of 0.004 on the first test suggests that that particular assay equipment had a lower detection limit of perhaps 0.003 (so an actual reading resulted), while the second test was run on different equipment that could not detect any level below 0.008. In all probability the second PSA was still at the same 0.004 level, but was now simply not detectable ( 0.004 is below the detection limit of 0.008) on the different assay. Anytime a < symbol appears with a reading we can simply substitute a "below the ability of our equipment to pick up". I think that many docs eliminate this by completely disregarding the third decimal place and if there is a "0.00something" result, they will as your doc wisely has done, simply report the test result as "something less than 0.01"
Post Edited (BillyMac) : 10/19/2013 5:52:39 PM (GMT-6)