(9) Autoimmunity Diagnosis Flashcards
Should diagnostic tests be used as screening tools?
No
They should be used to answer specific questions and/or to support a clinical diagnosis but not as screening tools
When is the ability of a diagnostic test to correctly discriminate between health and disease improved?
When they are used in the appropriate population
What is ‘sensitivity’?
Measure of how good the test is in identifying people with the disease
a/(a+c)
What is ‘specificity’?
Measure of how good the test is at correctly defining people without the disease
d/(b+d)
What is ‘positive predictive value’?
The proportion of people with a positive test who have the target disorder
a/(a+b)
What is the ‘negative predictive value’?
The proportion of people with a negative test who do not have the target disorder
d/(c+d)
What graph can be used to give you an idea as to whether a diagnostic test would be valuable?
A normogram
What is the likelihood ratio (LR)? (used in a normogram)
probability of finding in patients with the disease / probability of same finding in patients without disease
The higher the LR, the more useful the test
What are the 3 parts of a normogram that should be connected by a line?
- pre-test probability
- likelihood ratio
- post-test probability
Give a type of non-specific diagnostic test
Inflammatory markers
Give 2 types of disease-specific diagnostic test
- autoantibody testing
- HLA typing
Give examples of non-specific markers of systemic inflammation
- ESR
- CRP
- ferritin
- haptoglobin
- albumin
- complement
(acute phase response proteins)
What are acute phase response proteins?
Class of proteins whose plasma concentrations increase (positive acute-phase proteins) or decrease (negative acute-phase proteins) in response to inflammation
How is C-reactive protein (CRP) an inflammatory marker?
Produced by liver after infection/inflammation
Changes acutely and so useful to measure treatment within 24 hours
What is ESR and how is it an inflammatory marker?
Erythrocyte sedimentation rate
Measures viscosity of plasma - viscosity increases after an inflammatory response
ESR takes a while to fully resolve after the infection + treatment
Does albumin (an inflammatory marker) increase or decrease in acute inflammation?
Goes down
Synthetic capacity of liver reduces
What are origins of ANA (antinuclear antibodies)?
LE phenomena in 1948
dsDNA identified in 1957
Anti-SM in 1966
What was noticed when bone marrow was taken and looked at from an SLE patient?
The nuclei of the erythrocytes had been engulfed by neutrophils and macrophages
What was noticed when the serum of an SLE patient was analysed?
Antibodies against double-stranded DNA (dsDNA) were present - autoantibodies directed to the nucleus
What are some possible specific targets of autoantibodies?
- double stranded DNA (dsDNA)
- RNP (ribonuclear proteins, protein machinery that deals with dsDNA transcription and translation)
- Ro (RNP)
- La (RNP)
What does ENA stand for?
Extractable nuclear antigens
These include the things that autoantibodies might be specifically targeted to
When might you ask for an ANA detection test?
Patient with hair loss, mouth ulcers, joint pain etc. Diagnostic test to confirm suspicions of lupus
What does an ANA detection test do?
It looks for autoantibodies against the nucleus in the serum
Why are secondary antibodies used in an ANA detection test?
They are used for visualisation - fluorescently tagged
They recognise the constant fc region of the ig molecule
Can see it brightly where there are autoantibodies against the nucleus
What techniques are used to detect anti-dsDNA antibodies?
- crithida lucillae assay
- farr essay
- ELISA
What techniques are used to detect ENAs?
- immunoblots
- individual ELISAs
- combination of antigens
How many antibodies described in SLE?
> 100
What is a newer more modern method of ANA detection?
Multiplex assessment of non-organ-specific autoantibodies with a novel microbead-based immunoassay
Can detect up to 20 things in a single serum sample
What is the sensitivity and specificity of an ANA test for SLE?
sensitivity = >95%
specificity = 60%
What is the sensitivity and specificity of a dsDNA test for SLE?
sensitivity = 70%
specificity = 95%
What is rheumatoid factor (RF)?
RF is the autoantibody first found rheumatoid arthritis
It is an IgM, IgG or IgA directed against the Fc portion of IgG
Is RF diagnostic of rheumatoid arthritis?
Commonly found in rheumatoid arthritis but not diagnostic, sensitivity and specificity around 70%
What other diseases other than rheumatoid arthritis can rheumatoid factor be seen in?
Other diseases in which polyclonal stimulation of B cells is seen (chronic infections)
What is vasculitis and what can cause it?
Inflammation of the blood vessels
May be caused by immune complexes precipitating in the capillaries
What is a more specific biomarker for rheumatoid arthritis than rheumatoid factor?
Anti-CCP (ACPA)
Present in 95% of patients with rheumatoid arthritis
What are ACPAs?
Anti-citrullinated protein antibodies
They are auto-antibodies that are directed against peptides and proteins that are citrullinated. They are present in the majority of patients with rheumatoid arthritis
How does the sensitivity of ACPAs compare to RF?
Similar sensitivity
How are ACPAs useful?
They are a useful prognostic marker
Allows us to pick up RA much earlier in progression of disease
Can find patients even in pre-clinical stage
ACPA positive patients tend to have what?
More severe and erosive disease
And so more aggressive therapy can be used
What are ANCAs?
Anti-neutrophilic cytoplasmic antibodies
They are a group of autoantibodies, mainly of the IgG type, against antigens in the cytoplasm of neutrophil granulocytes and monocytes. They are detected as a blood test in a number of autoimmune disorders, but are particularly associated with systemic vasculitis
When were anti-neutrophilic cytoplasmic antibodies (ANCA) first described?
In 1982 as an incidental finding when using granulocytes as a substrate for ANA (GS-ANA)
In 1985, ANCAs were first described as an antibody specific for what?
Wegeners granulomatosis
As well as neutrophils, what can be used as a substrate for ANA?
Fibroblasts
used in SLE
What are the two types of ANCA?
- cytoplasmic (c)ANCA
- perinuclear (p)ANCA
What are cytoplasmic (c)ANCAs?
Antibodies directed to cytoplasm, sparing the nuclei
See granular fluorescence of neutrophil cytoplasm with nuclear sparing
What are perinuclear (p)ANCAs?
Antibodies directed to the nuclei, spring the cytoplasm
See apparent fluorescence of the nucleus only
How are neutrophils treated when being used as substrates for ANCAs?
Treated in a specific way using ethanol, allows the nucleus to be better visualised
What are the target antigens for cytoplasmic (c)ANCAs?
- PR3 (90%)
- azurocidin
- lysozyme (1%)
- MPO
What are the target antigens for perinuclear (p)ANCAs?
- MPO (70%)
- azurocidin
- B-glucuronidase
- cathepsin G (5%)
- PR3
Positive ANCA is extremely useful in suggesting the diagnosis in the proper clinical setting but what remains the gold standard for diagnosis in most cases?
Histopathology
Do negative ANCA assays exclude AASV?
No
10-50% of patients may be ANCA negative
Does persistence of ANCA in the absence of clinical indications of active disease indicate a need for continued treatment?
No
What does a reemergence of ANCA positive in a patient who was ANCA negative whilst in remission suggest?
A risk of disease flare
The temporal correlation between the return of ANCA and a disease flare is poor
Which autoantibody is specific for primary biliary sclerosis?
Anti-mitochondrial Ab
Which autoantibodies are found in autoimmune hepatitis?
Anti-smooth muscle
Anti-liver/kidney/microsomal (LKS)
What are the several types of autoantibodies in type 1 DM?
- islet cell antibodies
- anti-GAD65 anti-GAD67
- anti-insulinoma antigen 2 (IA-2)
- insulin autoantibodies (IAAs)
Autoantibodies in type 1 DM disappear with what?
With progression of disease and total destruction of B islet cells
What is the role of autoantibodies in diagnosis of type 1 DM?
- disease conformation
- 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
What is involved in the future of diagnostic testing in investigation of autoimmune diseases?
- cytokine determination in serum
- detection of antigen specific autoimmune T and B cells
- T-reg detection, measure of therapeutic response
- personalised medicine, genetic profiling to determine individual risk of the disease and to tailor the most appropriate therapy
