(gentle music)
- [Narrator] Pretest Probability of Lyme Disease.
Eight-year-old Sophia presents with a left knee
that has been swollen and mildly painful for three days.
Sophia is usually in good health
and has not had prior joint pain.
She lives in Pennsylvania
and enjoys playing outside.
She does not recall any tick bites,
and her mother does not remember seeing any ticks on Sophia.
Her vital signs are normal
and she has no rash.
Sophia's doctor examines the left knee
and notes a large effusion,
limited range of motion,
and associated erythema, warmth, and mild tenderness.
Sophia can still ambulate, but with a limp.
Sophia's presentation might be consistent
with Lyme arthritis,
and her doctor wonders if she should test for Lyme disease.
Before testing for Lyme disease,
clinicians should ask themselves two questions.
First, what is the pretest probability?
And second, what is the disease stage?
Pretest probability is defined
as the likelihood of disease
before a diagnostic test result is known.
In other words, what is the clinical probability
that a particular patient has Lyme disease?
Three questions can help you make this determination.
First, has the patient been in an area
where Lyme disease is common?
Second, was the patient likely exposed to ticks?
Third, does the patient have symptoms
that are characteristic of Lyme disease?
When the answer to all three questions is yes,
pretest probability is moderate to high
and testing might be helpful.
When the answer to one or more of these questions is no,
pretest probability is lower.
A patient with low pretest probability for Lyme disease
has an increased risk of a false positive test result,
which can lead to misdiagnosis.
Misdiagnosis can result in unnecessary treatment
and patient anxiety,
as well as failure to treat the true cause of illness.
Sophia is from Pennsylvania,
a state where Lyme disease is common.
Even though she doesn't remember a tick bite,
she might have been exposed to ticks,
given her active lifestyle.
Sophia's acute knee swelling
is characteristic of Lyme arthritis,
a manifestation of late disseminated Lyme disease.
Therefore, Sophia has a moderate to high
pretest probability for Lyme disease.
For patients with a moderate to high pretest probability,
diagnostic testing might be helpful.
However, the disease stage strongly affects
the sensitivity of serologic testing.
The recommended two-step serologic test for Lyme disease
is less sensitive at early stages of disease,
such as when a patient presents
with an erythema migrans rash,
which, on average, presents only seven days after infection.
Because serologic testing
depends on the detection of antibodies,
which can take several weeks
to develop after initial infection.
Because test sensitivity is poor
for patients with erythema migrans,
a manifestation of early localized Lyme disease,
treatment should be prescribed
without performing diagnostic testing.
The sensitivity of Lyme disease serology
is excellent for Lyme arthritis,
a manifestation of late disseminated Lyme disease.
Testing could be very helpful in Sophia's case.
A positive two-step serologic test for Lyme disease
would support a full course of antibiotic treatment
for Lyme arthritis.
A negative test would suggest an alternate diagnosis
and should prompt the clinician
to consider additional diagnostic evaluation.
Learn more about Lyme disease testing
while earning free continuing education credits.
Visit www.cdc.gov/lyme/healthcare.