The infected unfed ticks have few spirochetes in them. The numbers grow by two orders of magnitude as they feed - the reason why old bites are much more dangerous than the fresh bites.
Even smaller and less noticeable are the larvae. And I know I learned that they cannot transmit the disease, because they only do so, if they have biten an infected animal before - so I never worried about small ticks, but apparently new research indicates, that this is wrong.
"However, newer research suggests that some pathogens, including Borrelia miyamotoi, may be transmitted from the adult female tick directly to her offspring.
This means larval ticks may already be infected before their first bite."
https://danielcameronmd.com/larval-ticks-borrelia-miyamotoi/
Hey, labs have QC failures all the time.
Lyme post-exposure prophylaxis (PEP) is a single dose of doxycycline (200 mg for adults, or weight-adjusted for children) taken within 72 hours of removing a high-risk blacklegged tick bite to prevent Lyme disease.
However, guidelines universally recommend against testing ticks. For instance, IDSA says this:
> Knowing tick characteristics (ie, species, life stage, and an assessment of the degree of blood engorgement) is helpful for anticipatory guidance and in determining if antibiotic prophylaxis to prevent Lyme disease is appropriate [127]. Tick identification is available in most commercial laboratories and at some local health departments. Studies from the United States and Europe have shown that detecting B. burgdorferi sensu lato in Ixodes spp. ticks, however, poorly predicts either subsequent disease (0–12.4%) [126, 128–133] or asymptomatic seroconversion (0–4.7%) [126, 129, 130, 132, 134]. This is likely due to a variety of factors that influence the likelihood of transmission and the observation that most Ixodes spp. ticks discovered by patients have been attached for <48 hours [61, 62, 135].
(https://www.idsociety.org/practice-guideline/lyme-disease/)
The point is that the test is likely not useful to reliably inform any sort of prophylactic treatment, especially if the risk of transmission is high enough to warrant prophylactic treatment in the first place, in which case the PEP is indicated regardless of the tick test.
Sure, it modifies the prior probabilities, but what's the clinical significance of that?