Table II. Comparison of RF findings in 4074 samples that were testedfor RF both by ELISA and
conventional agglutination.
ELISA negative
ELISA positive
*
Total
Agglutination test
Negative
3604
(88.4%)
90
(2.2%)
3694
Positive~
55
(1.4%)
325
(8.0%)
380
' Positive refers to a titer of 0!::1 :40 in the RAPA test.
Total
3659
415
4074
*
Positive refers to elevation of IgM, IgG or IgA RF in the class specific ELISA.
elevation of only one RF dass.
Table III. Elevated rheumatoid factor classes in
Ascanbeseenfromtablellatotalof415samples 90samples that were negative when tested by
were positive in the class specific ELISA. Table IV agglutination but positive by ELISA.
shows that the most common RF pattern was an
isolated elevation of IgM RF (27%), followed by
elevationoflgM+IgARF(26%) andigM+IgG+IgA
RF (20%). The great majority of samples with
elevation of IgM RF were also positive by aggluti–
nation (Table IV). In contrast, the sensitivity of the
agglutination test was s50% forsamples that lacked
IgM RF (Table IV).
DISCUSSION
The main conclusion of this analysis is that a
satisfactory earrelation between agglutination and
ELISA is only found when IgM RF is elevated
(Table IV). Thus, the majority of samples with
elevation of only IgA RF and/or IgG RF were
negativewhen test
ed
by agglutinationwhilesamples
with an isolated elevation of IgM RF were usually
positive (Table IV).
As can be seen from fig. l only 3% of individuals
with isolated elevation of IgM RF have RA
compared to 32% of individuals with elevation of
both IgM RF and IgA RF and about two thirds of
seropositive RA patients have elevation of both
IgM RF and IgA RF [21] . As mentioned before
elevation of IgA RF has been associated with poor
prognosis and the development of bone erosions
126
Elevated
Numberof
RF class(es) :
samples
(%)
lgMRFonly
22 (24%)
lgG RF only
31
(34%)
IgA RF only
23 (26%)
lgM + lgG RF
3 ( 3%)
lgM + IgARF
6 ( 7%)
lgG + IgARF
5 ( 6%)
lgM + IgG + IgA RF
o
( 0%)
Total
90 (100%)
[6, 7, 16] while RA patients with elevation of only
IgM RF mayhave a relativelymild disease course
[7, 21]. Furthermore, most patients with milder
forms of arthritis and rheumatic eliseases other than
RA have elevation of only one RF class, mostoften
IgM RF.
It
is therefore important for the clinician
Klinis/c kemi
i
NortUn 4, 1994
