rheumatology diagnostics Flashcards
what are the main categories of tests done in rheumatotology?
Blood tests
Joint (synovial) fluid
analysis
Imaging tests
- X-rays
- Ultrasound
- CT
- MRI
what blood tests do you do in rheumatology?
basic ones: Full blood count (FBC) Urea and electrolytes (U&E) Liver function tests (LFT) Bone profile Erythrocyte sedimentation rate (ESR) C-reactive protein (CRP)
what happens to FBC in different types of arthritis?
inflammatory: Hb - down or normal MCV - normal WCC - normal PLT - normal or up
(Inflammatory can suppress the bone marrow, causing anaemia - low Hb)
osteoarthritis: Hb - normal MCV - normal WCC - normal PLT - normal
septic: Hb - normal MCV - normal WCC - increases PLT - normal or up
why are U&Es done in rheumatology?
Urea (U)
Creatinine (Cr)
Sodium
Potassium
Higher Cr = worse renal clearance (indicating kidney problem)
Relevance:
Rheumatological diseases can affect the kidneys
Examples:
a) Systemic lupus erythematous (SLE) -> lupus nephritis
b) Vasculitis -> nephritis
c) Chronic inflammation in poorly controlled inflammatory disease
-> high levels of serum amyloid A (SAA) protein -> SAA deposits in organs (AA amyloidosis)
Non-steroidal anti-inflammatory drugs (NSAIDs) (eg ibuprofen) can cause kidney impairment
why are LFTs done in rheumatology?
Bilirubin
Alanine aminotransferase (ALT)
Alkaline phosphatase (ALP)
Albumin
Relevance:
Disease modifying anti-rheumatic drugs (DMARDs) (eg methotrexate) can cause liver damage.
Key point: patients on methotrexate need regular blood tests (eg every 8 weeks).
Low Albumin: can either reflect problem of synthesis (in liver) or problem of leak from kidney (eg in lupus nephritis)
what is a bone profile?
Calcium
Phosphate (PO4)
Alkaline phosphatase (ALP)
nb also in LFTs – confusingly the source of ALP can be bone OR liver
Paget’s disease of bone: ALP ↑↑
Paget’s = disease caused by abnormality of high bone turnover.
Clinical features: bone pain, excessive pain growth, fracture through area of abnormal bone
Osteomalacia (soft bones due to vitamin D deficiency): ALP normal or ↑, Ca and PO4 normal or ↓
Osteoporosis (low bone density): usually calcium, PO4 and ALP normal
Dexa scans are needed for osteoporosis
why are ESR and CRP done in rheumatology?
Both ESR and CRP are useful markers of inflammation
However, ESR can be up for other reasons:
- Elevated immunoglobulin level
- Paraprotein (myeloma)
- Anaemia
- Tends to rise with age
Usually CRP is more specific for inflammation
Rule of thumb in SLE:
ESR usually high but CRP normal
Exceptions to the rule: CRP high in SLE if there is significant synovitis or there is an inflammatory pleural or pericardial effusion
If CRP in lupus, have a low index of suspicion for infection
what are the autoantibodies in rheumatoid arthritis?
1) Rheumatoid factor (RF)
Antibodies that recognize the Fc portion of IgG as their target antigen typically IgM antibodies i.e. IgM anti-IgG antibody !
Positive in 70% at disease onset and further 10-15% become positive over the first 2 years of diagnosis
2) Cyclic citrullinated peptides (CCP) antibodies
More specific than RF
Associated with worse prognosis
what are anti nuclear antibodies?
ANA
Antibodies directed at nuclear component of the cell
Non-specific:
Relatively common in general healthy population at low titre (level)
Prevalence of ANA increases with age in the general population
Sometimes transiently positive following infection
Use in rheumatology:
High titre ANA in combination with the correct clinical features may indicate one of the autoimmune connective tissue diseases (eg SLE, Sjogren’s syndrome, scleroderma)
Learning point: important to order this test judiciously. If you order it indiscriminately you will cause many healthy people to have an abnormal test result which will lead to anxiety and unnecessary referral to hospital and further investigation. Only order if you suspect autoimmune connective tissue disease clinically
what conditions would you order ANA for?
autoimmune connective tissue diseases
SLE: Arthritis Skin rash Mouth ulcers Kidney disease Haematological Pleural effusion Pericardial effusion
Scleroderma:
Vasculopathy (esp. Raynaud’s phenomenon)
Skin thickening
Organ fibrosis
Polymyositis:
Muscle inflammation
Weakness
High CK
Sjogren’s syndrome:
Dry eyes
Dry mouth
Extra-articular features
how do you interpret ANA results?
ANA interpretation:
- Strength of ANA is reported as maximal dilution at which it is still detectable
eg 1:80 (weak), 1:320, 1:640, 1:1280 (strong) - Negative test rules out SLE
- Positive test does not necessarily mean SLE, but suggestive IF there are other clinical and lab features to support the diagnosis. A stronger test is more likely to be clinically significant
what other tests should you order if ANA is positive?
ENA (extractable nuclear antigens): a panel of 5 autoantibodies
Ro Lupus or Sjogrens syndrome
La Lupus or Sjogrens syndrome
RNP Lupus or mixed connective tissue disease
Smith Lupus
Jo-1 Polymyositis
Double stranded (dsDNA) antibodies: highly specific for lupus, associated with renal involvement, useful for tracking lupus activity over time Complement levels C3 and C4: may be ↓ in active lupus
what is synovial fluid analysis ?
Obtained by aspirating fluid from a joint
Indications for joint aspiration:
a) Diagnostic: to obtain synovial fluid for analysis
b) Therapeutic: to relief symptoms (+/- concurrent steroid injection)
Two main diagnostic uses for aspiration:
- Suspected septic arthritis
- gold standard for diagnosis
- send for MC&S
- enables causative organism to be identified
- sensitivities from culture guide antibiotic choice - Diagnosing crystal arthritis
how does synovial fluid analysis work in crystal arthritis?
The diagnosis of crystal arthritis is made by aspirating fluid from the affected joint and examining it under a microscope using polarized light
Gout:
needle shaped crystals with negative birefringence
Pseudogout:
rhomboid shaped crystals with Positive birefringence
what are the key differences between septic and reactive arthritis?
septic arthritis:
synovial fluid culture - positive
antibiotic therapy - yes
joint lavage - yes for large joints
reactive:
synovial fluid culture - sterile
antibiotic therapy - no
joint lavage - no
