Rheumatoid factors: Quantitation,
Standardization,Clinical value and
east-considerations
Bjarne Falk Rangård, Dept. of Clinical Chemistry,Kolding Hospital, Denmar
k.
lntroduction:
In recent years many laboratories have changed
from qualitative and semiquantitativemeasuring of
rheumatoid factors to quantitative measuring. The
rationale behind this change is partiy improvement
of analytical performance and part!y the possibility
of using automated instruments and thereby reduce
costs. On the Scandinavian leve! the main choice
has been nefelometric/turbidimetric measurement
on standard clinical chemical analysers. In Den–
mark the clinical immunologkal departments have
ehosen the EUSAmethod for measuring RF-IgM.
Thereby we have created two kinds of quantitative
measurement for RF's in clinical practise, and we
have to consicter the standardisaHon problems
related to this new situation.
Principles of standardisation:
The basic principle of standardisation are
- to define the properties of the component you
wish to measure.
- construct a reliable measurement system
- produce a standard and choose an international
calibrator
WHO has recommended an international stan–
dard for measuring of rheumatoid factors . The
International
U
nit is definedas the activity contained
in 0.171 mg of the international reference prepara–
tion (ref 1).
The selected property of the component was the
agglutinability measured by assays of the Rose–
Waaler-type.
On the basis of this factor, it seems logical to
select an agglutination assay for quantitation of
rheumatoid factor. Considering the rheumatoid
factors of IgM-type it is rather simple to make
quantitative assays.
As
Iong as you use a direct measuring principle
90
(ref 2) like nefelometry/turbidimetry, you are able
to use the International arbitrary
U
nit
(IU)
without
violating any basic principle of unit definition.
If
you on the contrary choose a derived assay (ref
2) like ELISA or FIA, you may expect problems
with linearity, comparability and correlation.
If
you want to quantitate Rheumatoid factors of
other immunglobulin-classes (lgG, IgA) you have
to choose derived assays, but that is a completely
different situation.
Results from proficiency testing programs:
Our laboratory has participated in externa! quality
assurance programs: The LabQuality determina–
tion survey 1991 and 1992 and a Danish survey in
1992. (ref.3 and 4) Some extracts of results have
been published recent!y in this journal (ref. 5) but
some important facts have escaped the authors '
attention.
Among the partidpants in the 1991 Labquality
Survey were 22 laboratories using nefelometry/
turbidimetry and only 4laboratories using ELISA.
One-hundred and thirty laboratoriesusedan agglu–
tination method (qualitative or semi-quantitative).
In the 1992 LabQuality Survey the number of
laboratories using nefelometry/turbidimetry had
increased to 29 while only 3 laboratories used
ELISA. Furthermore there was no statistical
difference between nefelometry and turbidimetry
as expected in accordance with the property (ag–
glutinability) of Rheumatoid Factor.
There is no sense in using mean values for
comparison between methods of different princip–
les when one of the groups (ELISA) is of this size
(3 laboratories).
Klinisk kemi
i
Norden 3, 1994
