MCHC Blood Test: Why a High Reading Usually Means the Sample, Not You

A low MCHC (hypochromia) means the cells are paler than usual. The two common causes, iron deficiency and thalassemia trait, look almost identical on the red cell indices, so a low MCHC confirms what a low MCV and MCH already showed rather than choosing between them.

The numbers next to it start the sorting. Thalassemia trait usually keeps the red blood cell count normal or high and the RDW narrow; iron deficiency lowers the count and widens the RDW. Ferritin, usually checked with iron studies, settles it, and the cutoff depends on who is tested. The American Society of Hematology treats a ferritin of 30 ng/mL or lower as iron deficiency in adults, and 50 or lower in higher-risk groups such as people with heavy menstrual bleeding. When inflammation is present, it looks for a ferritin under 100 or a transferrin saturation under 20 percent.

Those cutoffs sit above many printed ferritin ranges. A woman with heavy periods whose report shows an MCHC of 31, an MCV of 76 and a ferritin of 40, within the lab's range, is iron deficient by the threshold for her group, so the ferritin value itself is worth asking for.

Iron deficiency without anemia is about twice as common as iron deficiency with it, so a normal hemoglobin next to a low MCHC is not an all-clear. If iron treatment starts, the NHLBI notes that oral iron often takes three to six months to rebuild stores, and a CBC repeated after a few weeks may barely change.

MCHC is also the slowest of the indices to move. It sits in a narrow band and shifts only after the MCV and RDW have already changed, which makes it a poor number to track on its own.

Only one of the two numbers behind MCHC is measured directly. Hemoglobin is read by passing light through a treated sample; the hematocrit is calculated from the cell count and average cell size. An MCHC above about 37 g/dL, which a normal red cell cannot produce, usually means one of those inputs was read wrongly.

A common example: a CBC drawn an hour after a fried breakfast shows an MCHC of 37.8 with a normal hemoglobin and MCV in someone who feels fine, and a repeat on another morning comes back at 34. Most high readings are settled by whether the lab re-ran the sample and whether the analyzer flagged clumping or a cloudy specimen.

If the high value holds on a fresh sample, two conditions are the usual explanations: hemolytic anemia, in which red cells break down early, and hereditary spherocytosis. In spherocytosis a fault in the cell membrane makes red cells round up into spheres, the same hemoglobin in less volume, so the concentration is genuinely higher. The spleen removes these stiff cells early, which causes anemia and jaundice.

MedlinePlus Genetics estimates hereditary spherocytosis affects about 1 in 2,000 people of Northern European ancestry, where it is the most common inherited anemia. About half of affected people develop gallstones, usually between late childhood and middle age, and about 75 percent of cases are inherited in an autosomal dominant pattern, so a parent or sibling often has it too. A high MCHC that repeats in a young person with gallstones, jaundice, an enlarged spleen or that family history is a reason for a blood smear and membrane testing.

MCHC is one of four red cell indices on the complete blood count, and the only one whose divisor, the hematocrit, is itself calculated. That makes it sensitive to errors in the numbers feeding it, which is exactly why labs use it as a check on their own results. The MCV vs MCH vs MCHC comparison works through the arithmetic with real counts, including why a high MCH so often comes with a normal MCHC.