2.6 Acceptable Variation: Gamma-glutamyl-4-nitroanilide Sub–
strate and Calibration using BCR Reference Material for GT
The analytical procedure is that recommended as a routine
method in 1975 by the Scanainavian Society for Clinical Che–
mistry ( 7).
Due to the lower salubility of the non-carboxylated donor
substrate gamma-glutamyl-4-nitroanilide, its final concentra–
tion was ehosen to be 4 mmol/L. As a consequence of the lower
donor substrate concentration, the accepter substrate gly–
cylglycine also had to belowered (75 mmol/L), and TRIS was
included to increase the buffer capacity. (7)
This gives a less optimized method than the IFCC method. The
Nordie Recommended (SCE) method gives catalytic activity con–
centrations that on the average are about 20 per
c~nt
lower
than those with the IFCC method, when patient's sera are as-
o
sayed at 37
c.
Quality control materials based on some non-
human enzymes give quite different ratios between the two
methods due to species differences with respect to substrate
specificity.
To obtain numerical comparability of the results with those
obtained with the IFCC reference method or the ECCLS routine
method described in 1.1 above, the variant procedure must be
calibrated using an appropriate, certified enzyme reference
material. The BCR Reference Material for GT (Product code CRM
nr 319 (8 a-c) appears suitable for this purpose. The commu–
tability of this preparation with clinical specimens when as–
sayed with the IFCC reference and the Nordie recommended met–
hods has been assessed, using procedures prescribed by the
ECCLS Subcommittee on Enzyme Reference Materials (9 section
II,2). This evaluation showedan average difference of< 2.5%
between results obtained by the IFCC Reference Method at
82
Klinisk kemi
i
Norden 2: supp/, 1990
