Last week I spoke with a prospective patient who was calling from Alberta. They had a negative elispot test, which is a T-cell assay for an inflammatory response specific to lyme, and was then told that based on this they absolutely did not have lyme. However, in briefly speaking with the patient, they were interested in seeking a second opinion because they had many of the symptoms, across multiple systems, which were migratory and intermittently severe.
No lab has a 100% sensitivity or likelihood of catching lyme disease. Coinfections could be at play, different strains or species of lyme, or the patient may just not have a very robust immune response against lyme, as the usual lyme testing is an indirect test, aimed at detecting the immune response. Further, for tests like the western blot, the CDC interpretation may be negative, but other published criteria might be positive for lyme, or at least hint that retesting or treating and then retesting would be a suitable course.
Often, if a western blot is equivocal or has some bands that are positive but not many, then after a discussion of the risks and possible benefits of treatment we might start on a course of treatment if the clinical picture matches lyme or a related condition. If there is no clear clinical response, then sometimes retesting the lyme western blot can be informative, looking for an increase in the bands seen.