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- [Narrator] Lyme disease serologic testing.
When testing for Lyme disease,
CDC recommends a two-step process
using FDA-cleared serologic tests.
These serologic tests are designed to detect antibodies
made by the body in response to infection.
Antibodies can take several weeks to develop,
so patients may test negative
if tested within the first few weeks after infection.
However, two-step Lyme disease serology is sensitive
and specific for patients with manifestations
of early-disseminated or late-disseminated Lyme disease.
When a patient with possible
early-disseminated Lyme disease,
or late-disseminated Lyme disease,
testing can be very helpful.
Again, two steps are required
and can be performed on the same blood sample.
If the first step is negative,
no further testing is recommended.
If the first step is positive,
or indeterminate, sometimes called equivocal,
the second step should be performed.
If the second step is negative,
the overall test is negative.
The overall test result is positive
only when the first and second tests are positive,
or for some tests, equivocal.
Some labs use Western immunoblots as second step test.
These tests give results for two types of antibody,
IgM and IgG.
To be positive, an IgM Western blot
requires reactivity to at least two of three specific bands
and an IgG Western blot requires reactivity
to at least five of 10 specific bands.
If the patient has been ill for more than 30 days,
only IgG results should be used.
Most infected patients will have mounted an IgG response
by 30 days after infection.
A negative IgG result after one month
suggests that the IgM results, if positive,
are likely a false positive.
Sometimes, labs that use a Western or immunoblot
as the second step test
will not report just a positive or negative result
for the immunoblot test.
Instead, they will report each individual band
that is present in the blot.
This can cause confusion for both patients and clinicians.
For example, here, only three of 10 specific bands
on the Western blot IgG test are positive.
The minimum criteria for a positive IgG Western blot
are the presence of five or more specific bands.
That means the overall result here is negative,
despite set several individual bands being present.
Western blots are not the only second step test available.
In 2019, the FDA cleared new tests
that use a second specific EIA
in place of the Western immunoblot assay.
These EIAs can be conducted either sequentially
or simultaneously on the same blood sample.
Some laboratories offer laboratory-developed tests
that are not FDA cleared.
CDC recommends against using these tests
as there is less assurance
regarding their clinical accuracy.
To summarize, two-step serologic testing
is the most reliable laboratory method
to diagnose Lyme disease,
but it has some limitations,
particularly during the first few weeks after infection.
Finally, don't forget that antibodies to most infections
persist in the blood for months or years
after the patient has recovered.
As a result, serial testing is not useful
as a test of cure,
and recurrent positive results are not evidence
of ongoing infection.
Learn more about Lyme disease testing
while earning free continuing education credits.
Visit www.cdc.gov/lyme/healthcare.