How Measles Is Diagnosed

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measles diagnosis
© Verywell, 2018 

Measles, also known as rubeola, is diagnosed by your symptoms, particularly fever, spots in your mouth, and the measles rash, as well as blood tests to confirm that you have measles and not one of several other conditions that can mimic some of its signs and symptoms, such as scarlet fever or mono. ​Nearly 246 people worldwide, usually children, die per day from this preventable disease. Measles-related deaths most frequently occur in developing countries with poor healthcare systems, but there are more and more outbreaks of measles in the United States and Europe due to lowered vaccination rates.

Self-Checks

If you or your child have developed the symptoms of measles, particularly a fever with the measles rash and bright red spots with white centers in your mouth (Koplik's spots), it's probably safe to say that you have measles. If you or your child are unvaccinated and have been exposed to someone with measles or traveled internationally, measles is even more likely.

Although many childhood illnesses are accompanied by a rash, the measles rash is pretty easy to differentiate from other rashes. Two to four days after the other symptoms begin, the rash starts on your hands and face, rather than on your trunk, as most other viral rashes do. It spreads down to the rest of your body, hands, and feet over the next few days. Another difference is that your fever usually persists and may get even higher throughout the course of the rash, which lasts for five to six days.

If you think you or your child has measles, call your doctor right away, but don't leave your house unless your doctor instructs you to. Considering that nine out of 10 unvaccinated people who are exposed to someone with measles get it too, you can end up putting a lot of people at risk. Your doctor may make special arrangements with you in order to make a diagnosis.

Labs and Tests

Your doctor will look for the very same signs and symptoms you did in your self-check when considering measles, as well as ask about your vaccination and travel history. Even if you've been immunized, it's possible to get measles, though unlikely as the measles, mumps, and rubella (MMR) vaccine is 97 percent effective. Suspected cases of measles are required to be reported to local health departments within 24 hours.

To confirm a case of measles, your doctor will likely run some blood tests and take a throat and/or nose swab. These tests detect the virus-specific indirect immunoglobulin M (IgM), an antibody that's usually present about three days after your rash appears. The antibody may not show up before then, will generally peak at day 14, and is usually gone about 30 days after your rash first appears. You may also have a urine sample taken since the measles virus can be present there as well.

Your samples may be sent to your state health department or the Centers for Disease Control and Prevention (CDC) where they may be cultured in order to determine the genotype of the virus. Genotyping can help discover or rule out links between measles cases and outbreaks, as well as distinguish whether you actually contracted the measles virus or you're reacting to a recent measles vaccination.

Differential Diagnoses

Your doctor may need to rule out other illnesses before diagnosing you with measles. This is especially true because measles is fairly rare in the United States and most doctors have never seen anyone with it. That said, measles is fairly easy to diagnose when you know what you're looking for and blood tests can confirm it. Other illnesses your doctor may rule out depend on how long you've been showing symptoms of measles.

Before the Rash Appears

During the few days before the measles rash appears, it may seem like you have a respiratory virus such as the flu, respiratory syncytial virus (RSV), or a rhinovirus, which is typically the virus responsible for the common cold. The Koplik's spots in your mouth can be mistaken for Fordyce spots, which are enlarged oil glands.

However, your fever is generally higher with measles than with other viral infections, which is one clue that you don't have a typical viral infection. Measles can also be mistaken for dengue fever before or after the appearance of the rash, but this can be ruled out with a blood test.

After the Rash Appears

Once the rash has appeared two to four days after your other symptoms, other illnesses that your doctor may want to rule out could include:

  • Other viruses that can cause rashes: These include chickenpox, roseola, rubella, hand-foot-and-mouth disease, and parvovirus, also known as the fifth disease. Usually, doctors can easily rule these other viruses out by how your rash looks, as well as your other symptoms.
  • Scarlet fever and toxic shock syndrome (TSS): These group A Streptococcus infections can cause rashes as well, but scarlet fever can be ruled out by your other symptoms; toxic shock syndrome is usually accompanied by low blood pressure and kidney problems.
  • Reaction to drugs: A rash that's caused by hypersensitivity to a certain drug can look like a measles rash, but your doctor can quickly rule this out if you haven't been recently exposed to any drugs. If there is still a question, it will become obvious once you quit taking the drug because the rash will go away once the drug is out of your system.
  • Meningococcemia: This bacterial infection, which is caused by the same type of bacteria that causes meningitis, may have very similar symptoms to measles, including a rash. A blood test will determine if you have this infection or measles.
  • Rocky Mountain spotted fever: This bacterial disease is spread by ticks and also has similar symptoms to measles, including a rash. It's diagnosed with a blood test or skin biopsy.
  • Infectious mononucleosis: Known to most of us simply as "mono," this viral infection may also present with similar symptoms, including a rash, particularly after you've taken certain antibiotics. A blood test can rule out or confirm mono.
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