Nevertheless, same analyzers in routine use today cannot per–
form the recommended two-step method. For determination of
the catalytic concentration of total CK, serum start may be
considered acceptable, provided that the user is
awa~e
of the
greater limitations in the direct measurable range of activi–
ties, and that the instrument's eonstant rate linearity fin–
ding pragrarorne is adequate.
2.7 Minimum Lag-Phase Time
At the IFCC and ECCLS standard sample volume fraction of
0.044 and under the standard reaction conditions a lag phase
of up to 60 s appears satisfactory for the majority of samp–
les at both reaction temperatures (26, 30).
2.8 Minimum Measurement Time
This is highly dependent on the catalytic activity concentra–
tion in the sample and the instrument. The only general gui–
deline that can be given is that the reaction should be moni–
tored for the time and number of data points required to ob–
tain acceptable precision at the upper reference limit with
the instrument's eonstant rate finding programme. Manual in–
struments may require longer measuring times at low catalytic
activity concentrations.
Provided that the spectrometer is capable of accurate absor–
bance measurements of up to 2.000, increases of absorbance up
to about 0.01 per seeond (about 40 ykat/L, 2400 U/ L) may be
measured. Beyond this rate samp1es must be diluted, with
NaCl, 150 mmol/L.
2.9 Sample Volume Fraction
The effects of varying the vo1ume fraction of sample are much
more pronounced than variations in the relative volumes of
reagents A and B. Only within a relatively narrow range is
there proportionality between the serum volume fraction and
the reaction rate (3, Sb -c, 11, 14, 21, 26).
26
Klinisk kemi
i
Norden 2: suppl, 1990
