undiluted sera.
Thus, this survey pointed towards a more com–
plicated reason for the discrepancies.
Considering the difference between the two
measurement principles, agglutination and bind–
ing to immobilised IgG, it canbe expectedthat both
reaction kinetics and affinity of the rheumafactor
would contribute to the result of the measurement.
Such variation in RF properties is certainly hidden,
disguised or levelied out, when controls are pre–
pared by serial dilution in buffer of a pool consist–
ing ofmany different serapositive patients. Conse–
quently, even if the two methods could be brought
in accordance with each other, these kinds of
external quality assessment surveys will in no way
guarantee, that the methods will also give the same
result in every single patient.
It
is our thesis, that this is not possible, and a
project with the aim to standardise the two methods
to yield similar results wouldbepredietablewasted
efforts.
To further substantiate our thesis, we selected 47
sera from patients positive for RF, and measured
the RF concentration in both assays.
The results are shown in Fig. l and 2.
It
can be
concluded that patients do have RF' s with differen-
ELISA, kiU/l
Klinis/c
kemi
i
Nortkfl l , 1994
ces in properties, which are measured with diffe–
rent sensitivities in the two assays.
It
is not known
what is the origin of these signals, but lgG-RF
interpherence, IgM-RF specificity andatfinity, Clq
and other unspecifically agglutinating factors may
be of varying importance in the two types of assay.
When we became aware, that we bad two diffe–
rent assays in the county, we renamed the compo–
nents they measure, to: S-Rheumatoid Factors, ac–
tivity, klU/1, and S-Rheumatoid Factors,
IgM-Antibody, klU/1. This is an unsatisfactory
solution.
suggestion on future procedure.
From a practical point of view a solution could be
to choose the agglutinationmethodas "themethod"
to detect RF. This would be feasible for countries
where the RF's are done in doctors' offices and
small labs just as a
+/-
reaction for diagnostic
purposes, but such a decision would present af
problem at least in Denmark and other countries,
which have changed to semi-centralised RF
quantification by ELISA.
As
can be seen from
Table Il, a decision to change to nephelometric
assays is not in accordance with the trend, and
many would object.
·····\.·····:t:-~---·····:t
....................... ............................ ------··················
·-
:
....
=·
+
-1
+
.
l.
.
•
-1
+
.
EUSA. kiU/l
21
