Verification study on the NORIP LDH reference intervals with a proposed new upper reference limit

We verified the lactate dehydrogenase (LDH) reference interval (RI) provided by the Nordic Reference Interval Project (NORIP). The serum LDH concentration was analysed on the Dimension Vista 1500 system with an IFCC method with a bias of +2.1 % and +2.7 % against NFKK Reference Serum X and ERM-AD453/IFCC, respectively, showing verification of transference of the NORIP RI. Selective data mining in clinical laboratory information systems for retrospective serum LDH test results was used to calculate an indirect RI. For the adult age group (18 to <70 years) the limits of the interval was 127 U/L (90 % CI: 123-132 U/L) and 240 U/L (90 % CI: 234-243 U/L). However, the NORIP upper limit for the adult age group is 205 U/L (90 % CI: 198-210 U/L). Accordingly, 25.1 % of LDH test results were above the NORIPs upper limit of 205 U/L. If LDH analysis was requested by the hospital's medical departments, outpatient clinics or general practitioners 29.2 %, 26.2 % and 20.9 %, respectively, were above the 205 U/L limit. Differences in transport time before centrifugation of blood, and different transport principles could not explain the relative high percent of test results above the NORIP 205 U/L limit. The indirect finding of an upper limit of 240 U/L (90 % CI: 234-243 U/L), and the relative high number of test result >205 U/L, suggests that the NORIP upper limit should be adjusted.