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, though that test is more accurate. Accurate diagnosis is important because treatment can help prevent possible long-term complications, such as heart and kidney disease.

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

Some harmful infections and medical conditions can also begin with symptoms that are similar to those of strep throat. While it is rare, strep throat itself can progress to cause serious complications. There are a few important issues that you should be on the alert for to make sure that there is no delay in the accurate diagnosis of your condition: 

• 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

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 laboratory tests that can detect either Streptococcus pyogenes itself or that detect markers that signal that the organism is present 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 then 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 then subjected to either 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. It is estimated that rapid antigen tests have about a 14 percent to 20 percent false-negative rate. For this reason, if there is a strong chance of strep throat infection, it is recommended that a negative result from a rapid antigen test be followed up with a throat culture (for confirmation).

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 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. Again, though, these complications are rare and seen in untreated patients.

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): also involves muscle aches and a sense of feeling run down
• Influenza (the flu)
• Pneumonia: an infection of the lungs that causes coughing, wheezing, and shortness of breath
• Bronchitis: an infection of the bronchi that causes a productive cough and may cause shortness of breath
• Mycobacteria infection: a lung infection that causes a cough and wheezing
• Fusobacterium necrophorum: an infection of the lungs that usually causes more severe symptoms than strep throat
• Systemic illness: can affect any organ of the body and may begin with symptoms similar to those of an early strep throat infection
• Appendicitis: may begin with a fever and aches, but usually causes abdominal pain as well
• Arthritis: symptoms similar to those of the complications of strep infection
• Primary HIV
• HSV
• Gonorrhea
• Diptheria
• Tularemia

Sources:

Atkinson TP, Centor RM, Xiao L, et al. Analysis of the tonsillar microbiome in young adults with sore throat reveals a high relative abundance of Fusobacterium necrophorum with low diversity. PLoS One. 2018 Jan 19;13(1):e0189423. doi: 10.1371/journal.pone.0189423. eCollection 2018.

Cohen JF, Bertille N, Cohen R, Chalumeau M. Rapid antigen detection test for group A streptococcus in children with pharyngitis. Cochrane Database Syst Rev. 2016 Jul 4;7:CD010502. doi: 10.1002/14651858.CD010502.pub2.