However, samples stored for longer periods, rnaera CK type 2,
and certain lyophilized controls (30,31) may require a longer
activation time, up to the full 300 s stated by the IFCC Re–
ference Method (14).
2.6 Start of Reaction with Serum
starting the reaction with serum has the following disadvan–
tages:
l.The CK
reaction proceeds at accelerating rate during the
combined times of activation and lag phase accumulating inhi–
bitory NADPH leading to high absorbance values. This decrea–
ses the analytical range of direct measurement of CK cataly–
tic concentrations to about 15
~kat/L
(900 U/L), i.e. about
half of that which may be measured in the substrate start
mode without dilution of samples.
2.When determining the residual catalytic concentration of CK
MB after immunainhibition of the CK M subuni':s (28,29):
a: The immunainhibition of CK M-subunits may proceed more
slowly and less completely in the presence of the sub–
strate, requiring up to lO min incubation in the presence
of creatine phosphate campared to about half that time in
its absence.
b: There is no possibility of measuring and substracting
sample residual adenylate kinase
(~~
EC 2.7.4.3) blank
reaction rates. Omission of this correction leads to fal–
sely increased CK MB results in cases with increased liver
AK or erythrocyte AK in hemelytic specimens (29).
3. Results of measurements made with serum start on patients'
sera may be about 2% lower than those made with substrate
start, and those made on certain control sera may be even
lower (19 a, 31).
Klinisk kemi i Norden 2: supp/, 1990
25
