Despite the frequent interchangeability of Hgb (g/dL) and Hct (%) by a ratio of ~1:3, directly-measured blood Hgb levels may be preferred for assessing the need for blood transfusion for at least 3 reasons:
First, in contrast to the widely-used automated measurements of Hct, Hgb is not affected by conditions that affect the size of the RBCs or the mean corpuscular Hgb concentration (MCHC). This is because the Hct is not a direct measure of Hgb; rather it’s the proportion of blood occupied by RBCs which, in automated systems, is derived by multiplying the number of RBCs by the mean corpuscular volume (MCV).1-3
This may not be a significant issue when MCHC is normal, but when MCHC is abnormal, HCT may not accurately reflect the blood Hgb concentration. For example, in patients with hypochromic iron deficiency anemia with RBCs containing less hemoglobin (ie, low MCHC), the Hct may overestimate blood Hgb levels. Conversely in hereditary spherocytosis with its attendant low RBC volume and high MCHC, the Hct may underestimate Hgb levels.
Second, Hct results may also be more subject to technical factors in the lab. For example, blood at room temperature between 6-24 h may be associated with RBC swelling and increased Hct without any change in its Hgb concentration.4
Finally, national and international guidelines on blood transfusion generally target Hgb, not Hct results.5-7
- Tefferi A, Hanson CA, Inwards DJ. How to interpret and pursue an abnormal complete blood cell count in adults. Mayo Clin Proc 2005;80:923-36. https://www.ncbi.nlm.nih.gov/pubmed/16007898
- Macdougall IC, Ritz E. The Normal Haematocrit Trial in dialysis patients with cardiac disease: are we any the less confused about target hemoglobin? Nephrol Dial Transplant 1998;13:3030-33. https://academic.oup.com/ndt/article-pdf/13/12/3030/9907456/3030.pdf
- Kelleher BP, Wall C, O’Broin SD. Haemoglobin, not haematocrit, should be the preferred parameter. Nephrol Dial Transplant 2001;16:1085-87. https://www.ncbi.nlm.nih.gov/pubmed/11328933
- Hayuanta HH. Can hemoglobin-hematocrit relationship be used to assess hydration status? CDK-237/vol 43 no.2, th. 2016 http://www.kalbemed.com/Portals/6/20_237Opini-Can%20Hemoglobin-Hematocrit%20Relationship%20Be%20Used%20to%20Assess%20Hydration%20Status.pdf
- Blood transfusion. NICE guideline, November, 2015. https://www.nice.org.uk/guidance/ng24/chapter/Recommendations#fresh-frozen-plasma-2 uk
- National Blood Authority: Australia. Patient blood management, November 2016. https://www.blood.gov.au/system/files/documents/nba-patient-blood-management-resource-guide-nov_2016_v3_sm_web_file.pdf
- Carson JL, Guyatt G, Heddle NM, et al. Clinical practice guidelines from the AAABB: red blood cell transfusion thresholds and storage. JAMA 2016; 316:2025-2035. https://www.ncbi.nlm.nih.gov/pubmed/27732721
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