123 Color index of blood
The colour index of blood (CI) is a traditional haematological index that shows the ratio between the amount of haemoglobin and the number of red blood cells. In other words, it shows how much haemoglobin an average red cell contains compared with a reference value taken as 1.0. It is widely used in Eastern European laboratory practice. In modern international practice the same information is given by the mean corpuscular haemoglobin (MCH): a colour index of 1.0 corresponds to an MCH of about 33 pg.
How it is calculated
CI = 3 × haemoglobin (g/L) / first three digits of the red cell count
For example, with haemoglobin 135 g/L and a red cell count of 4.50 ×10¹²/L: CI = 3 × 135 / 450 = 0.90. The same numbers give an MCH of 135 / 4.5 = 30 pg, and 30 / 33 ≈ 0.90. The two indices express the same quantity on different scales. The colour index is therefore exactly as reliable as the haemoglobin and red cell measurements it is based on.
Low colour index — hypochromia
A colour index below about 0.85 means that red cells contain too little haemoglobin. The most common cause is iron deficiency: chronic blood loss (heavy periods, gastrointestinal bleeding), poor intake or absorption, or pregnancy. Thalassaemia, anaemia of chronic inflammation and, rarely, lead poisoning or sideroblastic anaemia are other causes.
High colour index — hyperchromia
A colour index above about 1.05 usually reflects large red cells filled with haemoglobin. This is typical of megaloblastic anaemia due to vitamin B12 deficiency (including pernicious anaemia and conditions after stomach surgery or with stomach polyps or tumours) or folate deficiency. Alcohol and liver disease can also increase red cell size.
Normal colour index with anaemia
When haemoglobin and the red cell count are both low but the index is normal (normochromic anaemia), possible causes include acute blood loss, haemolytic anaemia (accelerated destruction of red cells), aplastic anaemia (reduced production), chronic kidney disease and chronic inflammation.
The colour index does not distinguish between small cells with little haemoglobin and normal-sized cells with low concentration. Modern analysers give MCV, MCH, MCHC and RDW, which together give a more precise description. The cut-offs 0.85–1.05 are conventional values from laboratory practice.
Consult a doctor if an abnormal colour index is accompanied by anaemia, tiredness, pallor, shortness of breath, numbness or tingling of the hands and feet, or signs of bleeding.
References
- Sarma PR. Red cell indices. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 152. ncbi.nlm.nih.gov/books/NBK260
- Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832–1843. doi:10.1056/NEJMra1401038
- Kaferle J, Strzoda CE. Evaluation of macrocytosis. Am Fam Physician. 2009;79(3):203–208. PMID: 19202968
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 9 May 2024