Flashcards in Diagnosis of Autoimmune Diseases Deck (21)
What are the general principles of diagnostic testing?
- Diagnostic tests should be used to answer specific questions and/or to support a clinical diagnosis, but not as screening tools
- The ability of the tests to correctly discriminate between health and disease is improved when they are used in the appropriate population
What is sensitivity?
Measure of how good is the test in identifying people with the disease.
What is specificity?
Measure of how good the test is at correctly defining people without the disease.
What is positive predictive value?
The proportion of people with a positive test who have the target disorder.
What is negative predictive value?
The proportion of people with a negative test who do not have the target disorder.
What are the different type of autoimmune diagnostic tests?
- Non specific
- Inflammatory markers
- Disease specific
- Autoantibody testing
- HLA typing
Which tests measure the non-specific markers of autoimmunity?
ESR - erythrocyte sedimentation rate (blood more viscous during inflammation)
CRP - C-reactive protein (more temporally sensitive)
Ferritin - increases during acute inflammation
Fibrinogen - increases
Albumin - decreases
Complement - increases
What is rheumatoid factor?
Antibody (IgM, IgG or IgA) directed against the Fc portion of IgG
What are the features of rheumatoid factor?
- Commonly found in rheumatoid arthritis but not diagnostic of the diseases (sensitivity and specificity around 70%)
- Can be seen with other diseases in which polyclonal stimulation of B cells is seen (chronic infections)
- High titers may be pathogenic in vasculitis
What are the features of Anti-CCP (ACPA)?
ACPA more specific (95%) for RA then RF
Similar sensitivity to RF
Useful prognostic marker
ACPA positive patients tend to have more severe and erosive disease
What is the clinical utility of measuring ANCA?
Particularly associated with Weneger's granulomatosis
Does not indicate a need for continued treatment
Reemergence of ANCA in a patient who was ANCA -ve whilst in remission suggests a risk of disease flare.
Histopathology remains the gold standard for diagnosis in most cases
Negative ANCA assays do not exclude AASV since 10%-50% of patients may be ANCA neg
Persistence of ANCA in the absence of clinical indications of active disease.
What is the role of testing for autoantibodies in T1DM?
To identify relatives and patients at risk of developing autoimmune diabetes
Negative predictive value of ICA and IAA is almost 99%
Increased risk of disease development with greater number of different autoantibodies present and younger age of patient
Name an autoimmune liver disease thought to be associated with anti-mitochondrial antibodies.
Primary biliary sclerosis
Name an autoimmune granulomatous vasculitis in which ANCA are detected.
Acronym for a very specific antibody in rheumatoid arthritis?
What is the relevance of rheumatoid factor?
Commonly found in rheumatoid arthritis but not diagnostic of the diseases (sensitivity and specificity around 70%)
Can be seen with other diseases in which polyclonal stimulation of B cells is seen (chronic infections)
High titers may be pathogenic in vasculitis
What is the relevance of anti-neutrophilic cytoplasmic antibodies (ANCA)?
Antibody specific for Weneger's granulomatosis.
In which liver disease would you detect Anti-mitochondrial Ab?
Primary biliary sclerosis
What antibodies are found in autoimmune hepatitis?
Anti-smooth muscle and anti-liver/kidney/microsomal (LKS) Abs
How are these antibodies detected?
Detected by immunofluorescence screening using rodent tissue block (oesophagus, liver and kidney) and antigen specific ELISA.