Viral Pharyngitis Workup

Updated: Jul 20, 2022
  • Author: KoKo Aung, MD, MPH, FACP; Chief Editor: Michael Stuart Bronze, MD  more...
  • Print

Laboratory Studies

The similarity of signs and symptoms of viral pharyngitis make a specific etiological diagnosis virtually impossible without various laboratory tests. In many circumstances, etiological diagnosis is of no practical use because it may not alter the treatment and prognosis. Viral cultures are not needed to diagnose pharyngitis other than in a research setting. [1]

The total WBC count may initially be slightly elevated without bandemia, followed by a decrease to fewer than 5000 cells/µL after 4-7 days of illness in about 50% of cases.

Atypical lymphocytosis is frequently associated with EBV and CMV infections.

Results from a rapid streptococcal antigen test and a bacterial culture of throat swab in viral pharyngitis may be positive (approximately 30% of patients with EBV infectious mononucleosis are colonized with group A streptococci).

Common cold

Specific virological diagnosis is unnecessary for practical purposes because it may not alter the management. Cultures of nasal secretions, serological tests, and polymerase chain reaction (PCR) techniques can be used for specific virological diagnosis. Rapid viral antigen detection tests are not sensitive enough to be useful.

EBV infectious mononucleosis

After week one of illness, peripheral blood film reveals relative and absolute lymphocytosis, with more than 10% atypical lymphocytes. Hemolytic anemia and thrombocytopenia secondary to anti-i antibodies are occasionally observed. Erythrocyte sedimentation rate (ESR) is elevated and liver function test results are mildly abnormal in about 90% of cases. Heterophile agglutination test (immunoglobulin M [IgM] antibody) results are positive with a titer of 40-fold or greater in 90% of affected adolescents and adults within the first few weeks after the onset of infectious mononucleosis symptoms. A mononucleosis spot test (Monospot) allows rapid screening for heterophile antibodies. Heterophile test results are usually negative in children younger than 4 years. Positive results for IgM antibody to viral capsid antigen and positive results for antibody to early antigen are useful to diagnose acute infection, particularly in cases that are heterophile negative.


Leukopenia and proteinuria are nonspecific findings in influenza. Virus isolation or detection of viral antigen in respiratory secretions is very useful to diagnose acute illness. Virus can be readily isolated from nasal swab specimens, throat swab specimens, nasal washes, or combined nose-and-throat swab specimens by inoculation of embryonated eggs or cell cultures. Rapid detection of viral antigen directly in respiratory secretions can be accomplished by immunofluorescent (IF) studies, time-resolved immunofluorescence assay (TRFIA), radioenzyme immunoassay, and enzyme-linked immunosorbent assay (ELISA).

PCR techniques have been described for rapid detection of influenza virus RNA in clinical samples. Serological tests can be used, but they are not helpful for diagnosis and treatment of acute disease secondary to delay in obtaining the antibody titers in convalescent sera. Serological tests are useful for epidemiological purposes. A rise in complement-fixing and hemagglutination-inhibiting antibody levels during the second week is considered diagnostic of acute infection.

Coronavirus infection

Nucleic Acid Amplification Tests (NAATs) such as reverse-transcription polymerase chain reaction (RT-PCR), and antigen tests of the upper respiratory tract specimens are used as diagnostic tests for diagnosis of COVID-19. The sensitivity and specificity of NAATs are very high. Positive NAATs indicate a current infection, or a recent infection with prolonged viral RNA detection. [6]

Antigen tests are immunoassays detecting the presence of a specific viral antigen.  They are more specific but less sensitive than NAATs. They have a rapid turn around time and therefore are used as home-based testing.  [6]

Enterovirus infection

Positive results on an enteroviral-specific reverse transcriptase-polymerase chain reaction (RT-PCR) test of throat swabs are diagnostic. Etiological diagnosis is not necessary for clinical purposes because it may not alter treatment.

RSV infection

RSV antigen in nasal secretions can be reliably detected with commercially available rapid tests.

CMV infection

A relative lymphocytosis is characteristic of acute CMV pharyngitis. Atypical lymphocytes may represent 10% or more of the total. CMV can be readily isolated from a throat swab. Positive results on the CMV-specific IgM antibody titers are diagnostic of acute infection. Results of heterophile tests are usually negative (heterophile-negative mononucleosis syndrome). A 4-fold or greater rise in antibody titers is confirmatory but useful only for epidemiological purposes.

Acute retroviral syndrome (primary HIV infection)

Serological test results for HIV are usually negative during the phase of acute retroviral syndrome because the test takes approximately 4 weeks for seroconversion. HIV RNA assay by PCR technique and p24 antigen assay can be used to help confirm the diagnosis. HIV viral load is usually extremely high. The peripheral blood picture may resemble infectious mononucleosis. Heterophile test results are usually negative (heterophile-negative mononucleosis syndrome).