02-28 Laboratory Testing in Rheumatology

Question 1

OBJECTIVE: ANA

• What is it?
• Disease associations?
• How is it measured?
• Patterns

Answer 1

Antibodies against one of many nuclear proteins

Disease Associations

• SLE - 98% sensitive
  
  • but highly non-specific

How It Is Measured

• old: look for LE cell
• IIF: HEp2 or rodent cells
• ELISA/EIA (enzyme immunoassay): automated, lower personnel costs

Five Patterns (see cocktail glass)

1. diffuse - useless
2. ** Peripheral** - due to Abs to either sDNA or dsDNA
   
   • dsDNA is specific for SLE
3. Speckled - ENA (extractable nuc. abs)
   
   • can be purified out for further testing
4. anti-centromere - specific for limitied scleroderma (CREST)
5. nucleolar - systemic sclerosis or SLE

ENA is a reflex panel done to follow-up on strongly positive ANA.

Question 2

OBJECTIVE: RF

• What is it?
• Disease associations?
• How is it measured?

Answer 2

It’s an IgM against Fc region of other Igs

Associate with everything under the sun

Measured by agglutination to latex particles coated w/ human IgG

• 1:80 is positive cut-off
• also get

Question 3

OBJECTIVE: Appropriate use of “tests of inflamm” (i.e. ESR and CRP)

Answer 3

ESR

• Normal values based on age & sex
  
  • ♂ upper limit of normal is age/2
  • ♀ (age + 10)/2
• seriously high if > 100; DDx
  
  • Malignancy
  • Inflammatory rheumatic disease
  • Infection
  • Renal disease
  • Less than 5% will have no explained cause
• Used to monitor RA, vasculitis, etc.

CRP

• If wicked high and ESR equivocal, think infection
• Named for reactivity to pneumococcal c-polysaccharide
• More expensive to get than ESR

Question 4

OBJECTIVE: ACPA vs. RF

Answer 4

ACPA as sensitive and is more specific for RA than RF

RF increases w/ age is elevated by myriad conditions

Question 5

OBJECTIVE: Review sensitivity and specificity (negative in health, positive in disease, Spin, Snout)

Answer 5

• Sensitivity => PID (positive in disease)
• Specificity => NIH (negative in health)
• S(P)in: when a specific test is positive you rule in a disease
• S(N)out: when a sensitive test is negative you rule out a dz

Question 6

OBJECTIVE: Concept of seropositivity and seronegativity

Answer 6

.

Question 7

PPV equation

Answer 7

TP/all positives

Question 8

NPV

Answer 8

TN/all negatives

Question 9

Modifiable risk factors that increase the amount of citrullinated proteins.

Answer 9

• smoking
• P. gingivitis

Question 10

Anti-Sm is specific for

Answer 10

SLE, also good prognostic sign that pt will have milder dz

small nuclear cytoplasmic antibody

Question 11

Anti-DNA topoisomeraes (Scl-70) is specific for?

Answer 11

Diffuse Cutaneous Systemic Sclerosis

• Worse prognosis than w/ Anti-Centromere
• more renal dz, ILD etc

Question 12

Anti-tRNA synthase is specific for?

Answer 12

DM/PM

Question 13

cANCA

Answer 13

Granulomatosis with polyangiitis 90% specific

a.k.a. PR3 ANCA

Question 14

pANCA

Answer 14

< specific than cANCA

• seen in microscopic polyarteritis, but also many others
• a.k.a. MPO ANCA

Question 15

Ro Abs and preg

Answer 15

can cross placenta and cause congenital heart block