13. Lab Testing Flashcards
Overview: what is critical to making a rheum diagnosis? in general, what is the role of lab testing?
History and Physical Exam are crucial: lab testing is expensive and should be done thoughtfully, based on the clinical diagnosis
What are the three categories of lab tests in general? A few examples for each category?
- Diagnostic (ANA, RF, ANCA, ACPA, HLA-B27)
- Prognostic (dsDNA Anti-SM, Jo-1 antibody)
- Evaluative (ESR, CRP)
When a SPECIFIC test is POSITIVE, you rule in or out a disease?
rule IN a disease (spin)
Specificity –> NIH (Negative in Health)
When a SENSITIVE test is NEGATIVE, you rule in or out a disease?
rule OUT a disease (snout)
Sensitivity –> PID (positive in disease)
ESR: what does it test for? What affects the rate?
tests for speed of sedimentation of RBCs (mm/hour).
Measures interaction of acute phase proteins and RBCs
Affected by shape of RBCs, number of RBCs, fibrinogen, hypergammaglobulinemia, anemia, age, pregnancy
Generally, what does an elevated ESR indicate?
a hypercoagulable state:
- malignancy
- inflammation
- infection
ESR: what rate is considered to be hign?
ESR > 100 considered sign of illness (malig, inflammation, infection)
ESR: what is its application in rheumatology?
- use to evaluate presenting symptoms in an overall clinical picture
- use to monitor disease activity (RA, vasculitis)
Case: 65 yo man with polymyalgia rheumatica (variant of RA) has decreased his dose of prednisone, but is now experiencing increased symptoms. Should he incr his dose of prednisone? what test might you run to check on the progression of his disease?
ESR to monitor his disease activity
An elevated CRP should make me think what?
think infection
CRP: what will cause it to be elevated?
any pathological condition associated with tissue destruction or inflammation.
CRP: where is it produced? what is it produced in response to?
- Produced in liver
- in response to IL-1, IL-6, TNF-alpha (pro-inflammaoty cytokines)
CRP: what would we expect this test to do after a surgery?
Levels generally rise 4-6h after tissue injury (ie surgery)
Levels then return to normal within a week assuming there are no post-op complications
CRP: application in rheumatology?
Use to evaluate an elevated ESR. Also use to monitor therapy.
If ESR is high, but CRP is normal, what is our conclusion?
NOT inflammatory because CRP is normal. But ESR is high so there is something going on: may be hypergammaglobulinemia since that would raise ESR but not CRP.
If I have an elevated CRP in a sick patient, what test should I probably also order?
blood cultures to further investigate possible infection
Rheumatoid Factor: what is it?
Auto-antibody IgM antibody directed against the Fc (constant) portion of IgA.
What are a few non-rheumatic conditions that will cause RF to be elevated?
- elev in some normal individuals (5%)
- elderly
- bacterial: endocarditis
- Viral: Hep C
ACPA: definition?
antibody to citrullinated proteins. these proteins occur all over the body, and are non-pathological. developing antibodies to them is the pathology and is unique to RA
ACPA: what are non-rheumatic risk factors for developing these?
- smoking (increases citrullination of proteins in lung)
- peridontal disease
ACPA: advantages over the RF assay?
- More specific for RA
- both sensitivity and specificity are higher
ACPA: what are its uses?
- identifying early inflammatory arthritis pts
- evaluating arthritis pts with negative RF
- evaluating pts who have positive RF but don’t have clinical signs of arthritis
Case: you have a patient who appears to have generalized osteoarthritis, with some joint deformities and some arthralgia. Her RF is elevated. How could you support your opinion that her RF is elevated due to age rather than development of RA?
ACPA is specific for RA, so get ACPA to rule it out.
HLA-B27: define? what disease is it correlated with?
MHC Class I antigen, marker for the shared epitope (HLA DR4)
Associated with ankylosing spondylitis
