How Scarlet Fever Is Diagnosed

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If you have concerns about scarlet fever but aren't sure how to get a diagnosis, a trip to your healthcare provider can help. There are simple tests that a healthcare professional can do, such as a strep test or a culture, along with an examination, to determine if your symptoms are caused by scarlet fever or something else.

Scarlet fever diagnosis
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Self-Checks/At-Home Testing

Scarlet fever is the presence of a rash on the body when you have an infection with Group A streptococcus bacteria causing a sore throat. Strep throat is the common name that is given for this infection. Scarlet fever simply means you or your child has a rash with strep throat.

Although this may seem like a simple thing that could be diagnosed at home, it is not.

There is no accurate way to determine if you or your child has strep throat at home, despite claims that "white patches" in the throat indicate strep. This is simply not true.

White patches may be present in the throat with strep, but they may also be present when a sore throat is caused by other bacteria or viruses and a person can have strep with no white patches. So, it's best to visit a healthcare provider to get a diagnosis.

Labs and Tests

A healthcare professional can perform quick, painless tests to confirm a scarlet fever diagnosis.

Rapid Strep Test

Rapid strep tests are—as the name suggests—very quick. Results are usually available in less than 10 minutes.

A swab is rubbed in the back of the oropharynx and then placed in a solution that indicates the presence, or lack of, the Group A strep bacteria. Although these tests are quick and convenient, they are not always 100 percent accurate.

Throat Cultures

Throat cultures are considered the gold standard for diagnosing scarlet fever and strep throat. The sample is obtained the same way as a rapid strep test—a swab that resembles a long Q-tip is passed over the back of the throat in the oropharynx. For this test, the sample is typically sent off to an outside lab where it is allowed to "grow" to determine if Group A streptococcus bacteria is present.

This test is more accurate but it takes longer, so many healthcare providers choose to use throat cultures as a backup diagnostic tool.

Often, a throat culture will be sent off if a rapid strep test is negative or if a child has a history of many false negative or false positive strep tests.

Differential Diagnoses

A person with a sore throat and rash can have any number of illnesses. A vast majority of rashes and sore throats are caused by viruses. However, if you or your child has this combination of symptoms, it's important to see your healthcare provider for an accurate diagnosis—even more so if a fever is present as well.

The rash of scarlet fever typically begins in the creases of the body—the neck, underarms, and groin—and then spreads to the torso and the rest of the body. It may start off looking like large flat red bumps and then will change to the appearance of red sandpaper. The cheeks often have a rosy appearance as well. 

Children with scarlet fever or strep throat that are left untreated may develop rheumatic fever, but it is rare in adults. Another potential complication of these illnesses is post-streptococcal glomerulonephritis, which is a very complicated sounding name for an inflammation of the kidneys after an infection with Group A strep. Your healthcare provider should be able to differentiate between all of these and provide appropriate treatment.

Frequently Asked Questions

  • How is scarlet fever diagnosed?

    Scarlet fever is diagnosed by lab tests that check for the presence of group A streptococcus. This includes a rapid strep test or a throat culture. Both tests use a swab to take a sample of secretions at the back of the throat. A rapid test returns the results in 10 minutes in the healthcare provider’s office, while a throat culture needs to be sent to a lab and can take a few days. 

  • Will scarlet fever test positive for strep throat?

    Yes, scarlet fever is caused by the same bacteria that causes strep throat—group A streptococcus. If a person has scarlet fever, it will be detected on a throat culture or rapid strep test.

  • What is the difference between strep throat and scarlet fever?

    Scarlet fever is essentially strep throat with a rash. Both are caused by the same bacteria and present with similar symptoms. The main difference is a red rash in scarlet fever.

  • What is the incubation period of scarlet fever?

    The incubation period for scarlet fever is between two and five days. 

1 Source
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Scarlet Fever: All You Need to Know

Additional Reading
  • Group A Strep | Post-Streptococcal Glomerulonephritis | | PSGN | GAS | CDC.
  • Group A Strep | Scarlet Fever | GAS | CDC. 

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.