Measles diagnosis in the United States runs into a statistical problem that has
nothing to do with the quality of the test.
**Because no assay is 100% specific, serologic testing of non-measles cases
using any assay will occasionally produce false positive IgM results.**
**In countries such as the United States where endemic circulation of measles
has been eliminated, most suspected cases are not measles.** When almost every
sample tested is negative, the small false-positive rate of a very good test
produces more wrong positives than right ones.
What the rash usually is instead
**Rash and fever are more likely due to a number of other rash-causing
illnesses:**
- Parvovirus B19
- Enteroviruses
- Human herpesvirus-6 (roseola)
Two more ways to get a positive that is not measles
Rheumatoid factor. Its presence **can also result in a false positive
IgM** — and **indirect enzyme immunoassays (EIAs) appear to be more affected
by RF than IgM capture assays**, so the choice of assay changes the answer.
Antibiotics. **Patients with throat or ear infections occasionally develop
a rash following administration of antibiotics.** A serum sample sent after
that rash may result in a false positive test. A child with a sore throat,
a prescription and then a rash is a common sequence, and it is a common way to
end up with a measles result nobody expected.
And the opposite error
**Serologic tests can also result in false-negative results when serum
specimens are collected too early with respect to rash onset.** Timing is part
of the test: IgM detected within the first few days of rash onset gives
presumptive evidence of current or recent infection, but too early is no
evidence at all.
What this means in practice
Because both errors are live, **suspected cases of measles must be laboratory
confirmed** rather than diagnosed from one result, and **the state or local
health department decides where specimens go** — the state laboratory, the
Vaccine Preventable Diseases Reference Center, or CDC. For equivocal cases,
a second-line assay may be necessary to determine immune status.
Confirmation uses more than serology: a capture IgM EIA built on
recombinant measles nucleocapsid protein, an indirect EIA for IgG, and
viral detection — culture, or real-time RT-PCR for measles viral RNA,
which looks for the virus itself rather than the immune response to it.
Source: Centers for Disease Control and Prevention.
许可协议： CC0 1.0（公有领域） · 改编自 www.cdc.gov
1
0
0
0
留言
