Myth: "IGeneX Lyme tests are always positive"
Dr. David Marz, the MD who had been diagnosed with ALS but got better with antibiotics after testing positive for Lyme, tested Lyme-negative several times on the much-maligned IGeneX tests, until he used antibiotics prior to testing. No Lyme test has no false negatives, but it is important to understand that there is no such thing as a false positive. Testing positive means that it is an absolute fact that living Borrelia spirochetes have once found their way into the bloodstream. And as long as it is not proven that they are fully eradicated from the CNS - something exceedingly hard to do since there is no reliable test to date that can exclude neuroborreliosis - it has to be assumed that a case of ALS could be a Lyme symptom. There are countless examples of people testing Lyme-negative on IGeneX tests.             
IGeneX stance is that Lyme testing according to the deliberately limited "Dearborn" criteria finds only 8% of Lyme infections. IGeneX's explanation :
The problem with "ordinary" Lyme tests is that they have been made so insensitive that they are worse than useless. Western Blot tests are usually specific to the B31 strain of Borrelia only, while there are many other strains. And they do not look for band 22, 23, 25, 31 and 34, even though those bands have been patented as the most specific bands for Borrelia.
Therefore, the very best Lyme tests that are commonly used suffer from 69% false negatives, and that is best-case. with ALS patients, that figure may well be much higher, because of documented OSP variation in the brain - an immune-privilleged site. Meaning, there are no IgM antibodies and very little IgG antibodies circulating any more, months or years after infection, but an active infection is firmly established in the CNS. A PCR of cerebrospinal fluid usually turns out negative as well, because spirochetes avoid this fluid like the plague, as Dr. Øystein Brorson has shown.
Tom Grier is a Microbiologist at the University of Minnesota Medical School, Duluth, Minnesota, USA. He explained why Lyme tests are notoriously unreliable - even less reliable than tossing a coin.  Dr. Grier wrote in even much deeper detail  about the deep inherent flaws of Lyme testing.
1 http://www.alsforums.com/forum/archive/index.php/t-1868.html (search for "Igenix")
14 http://www.arthritisinsight.com/archives/test12378.htm (search for "I tested negative by Igenex")