Direct and indirect estimation of reference limits for hematologic parameters in adults included in the HUNT study

Reliable reference limits for hematological parameters are essential for clinical interpretation. In this study, we estimated reference limits for hemoglobin, erythrocytes and MCH using direct and indirect methods on data from the Trøndelag Health Study (HUNT). Results from 53,591 men and non-pregnant women participating in HUNT4 survey were used to estimate reference limits using two indirect methods (refineR and TMC13). A healthy defined subset of 15,755 participants (29%) was used to establish reference limits using the direct method. Reference limits were estimated separately for men and women in three age groups. Due to limited sample size, only direct estimates were generated for participants ≥80 years. Among women the estimated 2.5 percentile of hemoglobin, using the direct method, was 119 g/L in those aged 19-49 years and 122 g/L in those aged 50-79 years. Corresponding estimates from the indirect methods were 120 and 121 g/L in women aged 19-49 years and 121 g/L in those aged 50-79 years. In men aged 19-49 years, all methods estimated the 2.5 percentile of hemoglobin at 137 g/L. In men aged 50-79 years the direct estimate was 135 g/L, while indirect methods estimated slightly lower values (132 and 133 g/L). Overall, indirect and direct estimates were largely consistent, with some exceptions. The observed age-related variation in reference limits for both sexes supports the need for age-specific reference intervals. We propose revised reference limits for hemoglobin based on these findings.