Influenza, RSV, and Strep Diagnostics

Flu Testing

Influenza or the flu is a contagious respiratory illness that infects the nose, throat, and lungs. Influenza A and B viruses routinely spread in people and are responsible for seasonal flu epidemics each year. Influenza A viruses are known to cause flu pandemics.

Symptoms

Influenza can cause mild to severe and even fatal illness. People with the flu may have some or all these symptoms: fever or chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue, and vomiting and diarrhea (more common in children).

Flu viruses seem to spread mainly by tiny droplets created when sick people cough, sneeze, or talk. One can spread flu to someone else while or before they know they are sick. People are most contagious in the first 3 to 4 days after illness onset, which occurs from one to four days after exposure.

Who and When to Test

Although flu testing is not required to make a clinical diagnosis, especially when seasonal influenza A and B viruses are circulating locally. However, flu testing can inform clinical management decisions about antiviral treatment, the need for other diagnostic tests, or initiate infection prevention and control measures.

Flu tests are recommended when admitting patients with suspected influenza to hospital. During outbreaks in hospitals, long-term care facilities, cruise ships, boarding schools, and summer camps, testing can help determine whether influenza is the cause.

Influenza Virus Tests

Common tests for flu are rapid influenza diagnostic tests (RIDTs) and rapid molecular assays.

Rapid Diagnostic Tests for Influenza

RIDTs can detect parts of the virus (antigens) that usually stimulate an immune response and produce results in about 10 to 15 minutes. RIDTs vary in their ability to detect flu viruses and appear to better detect flu in children than adults. During an influenza outbreak, a positive rapid flu test is likely to indicate infection.

Rapid Molecular Assays

Rapid molecular assays detect the genetic material of the virus, also produce results in 15 to 20 minutes, but are more accurate than RIDTs. These assays have high sensitivity (90 to 95%) and specificity. Several other more accurate and sensitive flu tests must be performed in hospital or public health laboratories. FDA-cleared rapid molecular assays produce results in approximately 15 to 30 minutes, and some are CLIA-waived for use at the point of care (POC).

Other Molecular Assays

Flu tests that use reverse transcription polymerase chain reaction (RT-PCR) or other molecular techniques identify the viral RNA or nucleic acids in respiratory specimens with high sensitivity and specificity. Some can detect and discriminate between influenza A and B infections and other tests can identify seasonal influenza A virus subtypes.

Culture and Serology Assays

Neither viral culture nor serologic (antibody) testing is recommended for diagnosing acute illness. The results of these tests are often used for surveillance, antigenic characterization of new seasonal influenza A and B virus strains, research, or public health investigations.

RSV Testing

Respiratory syncytial virus or RSV typically causes mild, cold-like symptoms from which patients recover in a week or two. But RSV can be serious for infants and older adults. It is the most common cause of bronchiolitis pneumonia in children in the United States under one year of age and can exacerbate asthma, COPD, congestive heart failure, and other conditions in adults.

Healthcare providers should check for RSV in patients with respiratory illness, particularly during the RSV season. In the United States, RSV usually circulates during fall, winter, and spring.

RSV Diagnostic Testing

Clinical symptoms of RSV are nonspecific and may overlap with other viral and some bacterial infections. Several types of laboratory tests are available to confirm an RSV infection and are performed using upper and lower respiratory specimens.

RSV Antigen Testing

RSV antigen testing is highly sensitive for children but less so for adults.

Rapid Diagnostic Tests for RSV

Real-time reverse transcriptase-polymerase chain reaction (rRT-PCR) is more sensitive than antigen testing. It should be used to test older children and adults, who typically have much lower viral loads than younger children.

Either method is effective for diagnosing RSV infection in infants and young children. The RSV sensitivity of antigen detection tests generally ranges from 80 to 90% in this age group. Healthcare providers should consult experienced laboratorians for more information on interpretation of results.

Less frequently, viral culture and serological assays for the presence of antibodies are used. The latter are usually reserved for research or surveillance studies.

Strep A Testing

Strep throat is a common type of pharyngitis in children from five through 15 years of age. It is rarely seen in children under three and is much less common in adults. It is necessary to perform a strep test to detect the presence of the bacteria in a patient.

The Streptococcus Group A bacteria are spread by coughing or sneezing, which produces small bacteria-laden droplets. People get sick after breathing in the droplets, drinking, or eating from the same dish as someone with the illness, or touching impetigo sores on the skin or contaminated with respiratory droplets and then touching their mouth or nose.

In general, strep infections are mild but painful. Common symptoms include a sudden onset sore throat, pain when swallowing, fever, red and swollen tonsils, petechiae (small red spots) on the roof of the mouth, and swollen lymph nodes. Other symptoms are headaches, stomach pains, nausea, or vomiting and a rash (scarlatina or scarlet fever). It takes two to five days for someone exposed to group A strep to become ill.

Rapid Diagnostic Tests for Strep A

Since strep throat cannot be diagnosed visually, a rapid antigen detection test (RADT) for strep or a throat culture is used. For rapid strep test procedures, a throat swab containing the sample is collected. If the rapid test is negative, a throat culture can be used for confirmation.