From the Guidelines
A negative measles serology but a positive measles swab indicates an early measles infection where the body hasn't yet produced detectable antibodies, which is a common occurrence in the first few days of infection. This discrepancy can be understood by recognizing that serologic tests detect the body's immune response to the virus, whereas the swab, typically using reverse transcription polymerase chain reaction (RT-PCR), directly detects the viral RNA. The positive swab confirms active infection, even if the serology is negative due to the timing of the test relative to the onset of infection.
Management and Prevention
Management of such a case includes:
- Immediate isolation of the patient to prevent transmission
- Supportive care with rest, hydration, and antipyretics for fever and discomfort
- Notification to public health authorities as measles is a reportable disease
- Vitamin A supplementation may be recommended, particularly for children and those with vitamin A deficiency, as it has been shown to reduce the severity of measles and its complications 1
- Close contacts should be assessed for immunity and offered post-exposure prophylaxis with MMR vaccine within 72 hours or immunoglobulin within 6 days of exposure if eligible
Follow-Up
Serologic testing should be repeated in 1-2 weeks, as antibodies typically develop within this timeframe, which would provide confirmation of the diagnosis and document immunity. It's also important to note that false-positive serologic results can occur, especially as measles becomes rare in areas with high vaccination coverage, and confirmatory testing with more specific assays may be necessary in some cases 1.
Vaccination and Immunity
Vaccination against measles is highly effective, with two doses of measles vaccine inducing serologic evidence of measles immunity in greater than 99% of persons if the first dose is administered no earlier than the first birthday 1. While immunity from vaccination is probably lifelong for most persons, there can be rare instances of secondary vaccine failure, though this has little effect on measles transmission and outbreaks 1.
Given the provided evidence, the key takeaway is the importance of interpreting negative serology in the context of clinical presentation and PCR results, especially in the early stages of infection, and managing the case accordingly to prevent further transmission and ensure the best outcomes for the patient and public health.
From the Research
Interpretation of Measles Serology and Swab Results
- A negative measles serology (serologic test for measles) but positive measles swab (reverse transcription polymerase chain reaction (RT-PCR) for measles) can indicate several scenarios:
- The individual may have received the measles, mumps, and rubella (MMR) vaccine recently, and the positive RT-PCR result is detecting the vaccine virus 2
- The individual may have a false-negative serology result, which can occur due to various factors such as low antibody levels or improper test performance
- The individual may be in the early stages of measles infection, and the serology test has not yet detected the antibodies
Factors Influencing Serology Results
- The timing of vaccination can affect serology results, with some individuals experiencing primary or secondary vaccine failure 3, 4
- The type of test used can also influence results, with different tests having varying levels of sensitivity and specificity 5
- Individual immune responses can vary, with some people producing lower levels of antibodies or experiencing waning immunity over time 6
Implications of Positive RT-PCR Results
- A positive RT-PCR result in an individual with a negative serology result may not necessarily indicate active measles infection, especially if the individual has recently received the MMR vaccine 2
- Public health responses to positive RT-PCR results should take into account the individual's vaccination history and risk factors for acquiring measles 2