How Strep Throat Is Diagnosed

Diagnosis of strep throat cannot be based solely on clinical signs and symptoms, and additional tests are required for positive identification of the Streptococcus pyogenes bacteria that cause the condition. A rapid strep test can yield same-day results, but it may take a couple of days to get the results of a throat culture, which is more accurate. A definitive diagnosis is important because treatment can help prevent possible long-term complications, such as heart and kidney disease.

Self-Checks

The early signs and symptoms of strep throat overlap with those of most upper respiratory infections. While you can't diagnose yourself with strep, you can look for the following, which may hint that strep throat could be affecting you or your child:

• Swelling in the back of the throat
• White spots or white patches on the back of the throat
• Small red or purple spots inside your mouth

There are a few important symptoms that could indicate a persistent or worsening infection:

• High fever (above 101 degrees)
• Persistent fever (lasting longer than 24 hours)
• Rash on the chest, arms, or neck
• Redness on the face
• Trouble breathing
• Decreased amount of urine; change in the color of urine

Labs and Tests

Because strep throat causes signs and symptoms that are so similar to those of viral infections and other bacterial infections, the best way to confirm the diagnosis is with a laboratory test. Tests can detect either the Streptococcus pyogenes organism or markers that signal the presence of the organism in the back of your throat.

Throat Culture

A throat culture has long been considered the diagnostic gold standard of strep throat infection, with the main disadvantage being a 24- to 48-hour delay in culture results.

The throat culture is performed by collecting bacteria from the back of the throat using a long swab. The sample is placed on special plates made of sheep blood mixed with bacteria growth media (called sheep blood agar plates) and allowed to grow in a lab.

Streptococcus pyogenes has a unique feature that allows it to break open red blood cells. When the bacteria are grown on these plates, there will be a clearing of red blood cells on the petri dish if Streptococcus pyogenes bacteria is present in the sample.

Rapid Antigen Detection Tests

A rapid antigen test is typically done in the doctor's office and is also performed by swabbing the back of the throat. The bacterial swab is subjected to enzymes or acid to extract parts of the Strep pyogenes bacteria, which are called antigens. Positive or negative, you should get the results in about 10 to 20 minutes.

Rapid antigen tests have much lower sensitivity than throat cultures, which means that they are more likely to mistakenly miss strep infection than throat cultures are.

Rapid antigen detection tests are not associated with a false positive rate, which means that if they come back positive, there is high certainty that you have a strep throat infection. 

Imaging

Imaging tests are not routine in the diagnosis of uncomplicated strep throat infection. However, tests such as X-ray, CT, MRI, or ultrasound may be necessary to evaluate serious complications of strep throat infections, such as kidney or heart involvement. These complications are rare and primarily seen in untreated patients.

Differential Diagnoses

Most upper respiratory infections can begin with the same signs and symptoms as strep throat. Here are some of the illnesses that are considered when you are being evaluated for a possible strep throat:

• Epstein-Barr virus (mononucleosis): This infection also involves muscle aches and a sense of feeling run down.
• Influenza (the flu)
• Pneumonia: This is a lung infection that causes coughing, wheezing, and shortness of breath.
• Bronchitis: An acute or chronic infection or inflammation of the bronchi causes a productive cough and may cause shortness of breath.
• Mycobacteria infection: This lung infection causes a cough and wheezing.
• Fusobacterium necrophorum: This infection of the lungs usually causes more severe symptoms than strep throat.
• Systemic illness: It can affect any organ of the body and may begin with symptoms similar to those of an early strep throat infection.
• Appendicitis: This condition may begin with a fever and aches, but usually causes abdominal pain as well.
• Arthritis: This chronic inflammatory disease causes symptoms similar to those of the complications of strep infection

Less often, primary HIV, HSV, gonorrhea, diphtheria, or tularemia can cause symptoms similar to those of strep throat.