Interpretations of laboratory results
Immunofluorescent test
The test are carried out on respiratory secretions or swabs from vesicular lesions. It is an antigen detection method. If antigen is detected with this method it indicate infection with that specific pathogen. Unlike virus isolation method negative results can exclude infection as the quality of the sample can be assessed with this technique.
IF technique is available for
HSV/ VZV
Influenza A/B
Parainfluenza
Respiratory Syncytial Virus
Adenovirus
If inadequate number of respiratory epithelial cells are observed, it is reported and it suggest that the quality of the specimen is not good and infection can not be exclude even if the result is negative.
Antigen detection
These method indicate presence or absence of the virus.
Antigen detected or positive means that the virus is present. It indicate current infection.
Antigen detection is available for a number of viral aetiologies
Viruses that causes diarrhea
Respiratory Viruses
HIV (P24 antigen)
Hepatitis B virus (HBsAg)
Unlike the IF, negative results on respiratory secretions does not exclude infection.
Serological tests
HSV antibody
HSV IgM HSV IgG
Primary infection + +
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Non-primary infection _ +
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Reactivation/ re-infection _ +
HSV IgM can be detected at low level in patient with recurrent infection. For this reason the test is not helpful to identify patient who has primary infection.
VZV antibody
VZV IgM VZV IgG
Chickenpox + +
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Shingles _ +
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Past infection/ Immunisation _ +
VZV IgM can also be detected in small number of patients with shingles or recurrent herpes.
EBV antibody
EBV VCA IgM EBV VCA IgG EBNA IgG
Acute/ recent infection + + _
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Past infection _ + +
All the three markers could be detected at the same time and it may indicate infection that occurred 8 weeks before the test or reactivation of EBV in immunocompromised patients or elderly patients.
CMV antibody
CMV IgM CMV IgG CMV IgG avidity
Acute/ recent infection + + Weak
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Past infection _ + Strong
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Reactivation _ + Strong
CMV IgM at low level can be detected in reactivation or re-infection. In pregnant women where primary CMV infection is the main causes of congenital abnormality, CMV IgG avidity test may provide supportive evidence to exclude primary infection. High avidity excludes recent infection.
HHV 6 antibody
HHV 6 IgM HHV 6 IgG
Acute/ recent infection + +
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Past infection _ +
The test for HHV 6 antibody is available only in a few laboratory.
Measles antibody
Measles IgM Measles IgG
Acute/ recent infection + +
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Past infection/ immunisation _ +
Mumps antibody
Mumps IgM Mumps IgG
Acute / recent + +
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Past infection/ immunisation _ +
Rubella antibody
Rubella IgM Rubella IgG Rubella IgG avidity
Acute/ recent + + Weak
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Past infection/ immunisation _ + Strong
In pregnant women additional test for Rubella avidity is required as low level of Rubella IgM can be detected in some patients with re-infection and in some as non-specific.
HIV antibody
HIV antibody p24
No evidence of HIV infection -- -
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Consistent with HIV infection + +
Hepatitis A
Hepatitis A IgM Hepatitis A IgG
Acute/ recent infection + +
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Past infection/ immunization - +
Hepatitis B
HBsAg anti-HBs HB core IgM / IgG HBe Ag anti-HB e
Acute infection + - +/+ + or - + or –
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Chronic infection + - -/+ + or - + or –
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Past resolved - + -/+ - +
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Post vaccination - + -/- - -
Hepatitis D
HBsAg HBcore IgM/IgG HDV IgM HDV IgG
Co-infection + +/+ + +
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Super-infection + -/+ + +
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Past infection + or - -/+ - +
Hepatitis C
Anti-HCV HCV PCR
Current infection + +
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Past resolved infection + _
Hepatitis E
Hepatitis E IgM Hepatitis E IgG
Acute/ recent infection + +
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Past infection/ immune - +
CFT interpretation
Suggestive of infection if the titre is 1/128
Indicates infection if the titre is greater than 1/128
Influenza A
Influenza B
Parainfluenza 1-3
Adenovirus
RSV
Infections with these viruses can be diagnosed with CFT. It is cheap.
Molecular tests
PCR
Qualitative DNA/RNA PCR
Viral RNA or DNA Detected or Positive.
This indicate the presence of the Virus, but it does not necessarily mean that it is the causes of the clinical problem.
If the virus is well known to cause the clinical problems experienced by the patient, it can be considered to be the causative agents.
The qualitative methods are used in patients who have follow up after exposure. It is also used for diagnostic purpose.
At present qualitative PCR/molecular test is available for
Herpes simplex
Varicella Zoster
Norovirus
Respiratory Syncytial Virus
Influenza A/ B virus
Parainfluenza
Adenovirus
Rhinovirus
Human metapneumovirus
Parechovirus
Enterovirus
Measles
Rubella
Mumps
Parvovirus
Papillomavirus
HIV
HTLV
Hepatitis C
Hepatitis E
Hepatitis D
In some specialist laboratories, they can perform qualitative PCR for
Flavivirus
Avian Influenza
SARS
Quantitative DNA/RNA PCR
Viral load are used to monitor patients who are known to have an infection with specific viral aetiology before or after therapy
The lower limit of detection for different viruses is different. For HIV, it is 50 copies/ml or 400 copies/ml depending on the kit used to measure the viral load. For most herpes viruses, it is 2000 copies/ml.
Viral load < 50, <400 or <2000 copies means that the virus was not detectable with the method used, but it does not mean that there are no virus particles.
For any herpes viruses, viral load greater than 10,000 copies can be considered to be significant treatment or intervention is required especially if the patients is considered to be immunocompromised.
Quantitative viral load should only be used on blood and CSF samples. Some laboratories do the quantitative tests on other samples and tries to interpret the result. One can argue that the viral load could vary depending on the water consumed in cases of urine samples or on the amount of fluid injected into the alveolar space in broncheoalveolar lavage.
Quantitative tests is available for
CMV
Adenovirus
HHV 6 & 7
EBV
HIV
Hepatitis B
Hepatitis C
BK virus
JC virus