1) Physiologic changes in the concentration of RF
2) Pathologic changes in the concentration of RF
a) Increased RF in:
Endocarditis
Rheumatoid arthritis
Sarcoidosis
Sjögren’s syndrome
SLE
Tuberculosis
Viral infections
Waldenström’s disease

Case study
65-year-old patient; for the past 6 weeks bilateral painful swelling and hotness of wrist and shoulder joints, metacarpophalangeal and proximal interphalangeal joints, and right ankle; weight loss of  5 kg during this time; fever, sweating at night; general malaise

Differential diagnosis
Vasculitis?
Sistemic lupus erythematosus?
Acute viral infection?
Rheumatoid arthritis?

Laboratory finding
ESR                 90/146 mm(Westergren method)
Hb                   100 g/L
WBC               13600 x 106/L
Iron                 12,7 mmol/L
AST                 50 U/L
ALT                 73 U/L
gGT                 87 U/L
CRP                8.5 mg/L
Albumin         48.7%
a1-glob          6.1%
a2-glob.         9.3%
b-glob.           12.5 %
g-glob.           23.4 %
RF                  1:1024 (aglutination test)
ANA                pos. (1:160
anti-DNA       neg