High MCV blood test: what it means and what to check next

Your blood count flagged the MCV as high, and the number itself already narrows the search. MCV is the average size of your red blood cells, and red cells grow larger for two broad reasons: either the marrow cannot copy DNA fast enough to divide on time (B12, folate, some drugs), or something else changes the cell membrane or the age mix of cells in your blood (alcohol, liver, thyroid, a burst of young cells). The first group can push MCV much higher than the second, which is why a reading of 103 and a reading of 121 lead to different next steps. This page explains what a high MCV can mean; it cannot tell you which cause applies to you, so take the result and these questions to the doctor who ordered the test.

MCV is reported in femtoliters (fL), a tiny unit of volume. The usual adult range is about 80 to 100 fL, and newborns normally run higher ². Macrocytosis, the medical name for large red cells, is generally defined as an MCV above 100 fL and affects about 3% of the general population ¹, with studies ranging from 1.7% to 3.6% ³. Most people learn about a high MCV by chance on a routine test, often with a normal hemoglobin ¹. For what MCV measures and how it fits the rest of the count, see the main MCV page and the CBC explained.

How high is your MCV?

The height of the MCV is the most useful single clue. Alcohol usually keeps MCV below 110 fL ¹, and so do most other non-megaloblastic causes such as liver disease, hypothyroidism and many medicines. Large oval cells above 115 fL on the smear, with hypersegmented neutrophils, suggest a megaloblastic cause such as B12 or folate deficiency ³. Treat the bands below as likelihoods: any cause can appear at any level, and a lab's own upper limit may sit at 96 or 98 fL.

If your MCV is borderline, see MCV 97 and MCV 98. If it is below 80 fL instead, the causes run the other way; see low MCV.

High MCH too? Why the two rise together

MCH is the average amount of hemoglobin in one red cell, measured in picograms (pg). A bigger cell holds more hemoglobin, so MCH climbs alongside MCV, and a high MCH next to a high MCV is the same finding counted twice. If both are high and your hemoglobin is normal, that is macrocytosis without anemia, which usually traces to alcohol, a medicine, thyroid, liver or an early B12 shortage. A high MCH with a normal MCV is a different question, covered on the MCH page; the MCHC page covers the concentration of hemoglobin inside the cell.

The common causes, from most to least likely

Doctors split macrocytosis into two families ³. Megaloblastic causes (from the Greek for "big cell") slow the marrow's DNA copying, so each cell keeps growing while it waits to divide and comes out large and oval. Non-megaloblastic causes enlarge cells by other routes, such as fat changes in the cell membrane or a higher share of young cells, and usually give smaller rises.

Megaloblastic: DNA copying is slowed

• Vitamin B12 deficiency: pernicious anemia (an autoimmune loss of the stomach factor that absorbs B12), stomach or bowel surgery, long-term acid-suppressing medicines, metformin, or a diet with no animal foods. See B12 deficiency.
• Folate deficiency: poor diet, heavy drinking, malabsorption such as celiac disease, pregnancy, and some medicines. See folate deficiency.
• Drugs that block DNA synthesis: hydroxyurea, methotrexate, azathioprine, zidovudine, and antiseizure medicines such as phenytoin and valproate ^{1 3}.

Non-megaloblastic: other routes to larger cells

• Alcohol: the cause in as many as 80% of patients in some populations, and it usually keeps MCV below 110 fL ¹. A raised GGT, a liver enzyme that rises with drinking, often sits alongside it, and MCV can stay high even when B12 and folate are normal.
• Liver disease, including fatty liver, which changes the fats in red cell membranes ¹.
• Hypothyroidism, an underactive thyroid ¹; see how to read your thyroid panel.
• A surge of reticulocytes (young red cells, which are larger than mature ones) after bleeding or hemolysis, the early destruction of red cells ¹.
• Myelodysplastic syndrome, a marrow disorder mostly seen in older adults, usually with other low counts ¹.
• Smoking, which can raise MCV slightly.

Normal or false readings

Newborns have a high MCV that falls over the first months ², and MCV can drift up slightly in pregnancy. Analyzers can also overstate MCV: cold agglutinins (antibodies that clump red cells in a cooled sample), marked hyperglycemia and a very high white count can all raise the reading ¹, as can a sample left standing too long before testing. A repeat test on a fresh sample is a reasonable first step when the result does not fit the picture; one reading rarely tells the whole story.

Medicines and supplements that bear on MCV

Strongest evidenceMetforminThe link between long-term use and low B12 is well established, one route to a rising MCV; ask for periodic B12 checks.

Strongest evidenceVitamin B12Doctors use B12, by tablet or injection depending on the cause, to treat a confirmed B12 deficiency, and MCV usually comes down once the deficiency is corrected.In our shop

Strongest evidenceFolateDoctors use folate to treat a confirmed folate deficiency, once B12 deficiency has been excluded or treated.

In the Diabetes Prevention Program Outcomes Study, each additional year of metformin use was linked to a 13% higher risk of B12 deficiency ⁶, which is why people on metformin are advised to have B12 measured from time to time. If you take metformin and your MCV is high, do not stop it on your own; ask your doctor about a B12 test.

Read MCV with the rest of your blood count

MCV almost never stands alone on a report. The markers printed next to it, and the liver panel if you had one, usually point to the family of cause faster than MCV alone. AST and ALT, mentioned below, are two liver enzymes.

The tests that find the cause, in order

A history of drinking, medicines and diet, a B12 level, a reticulocyte count and a blood smear help pin down the cause in most people ¹. When the smear shows large oval cells and hypersegmented neutrophils (white cells whose nucleus is split into too many pieces, a classic sign of low B12 or folate), B12 or folate deficiency is the most likely cause ³. These are the tests a doctor will usually order, roughly in this order.

1. 1Repeat the count with a smearConfirms the high MCV on a fresh sample and shows whether the cells look megaloblastic.
2. 2First-line blood testsVitamin B12, folate, TSH for thyroid, a liver panel including GGT, and a reticulocyte count.
3. 3If B12 is borderlineNICE treats total B12 below 180 ng/L (the same number in pg/mL, the unit on most US reports) as deficient and 180 to 350 ng/L as indeterminate ⁵. In that grey zone, active B12 (holotranscobalamin) or methylmalonic acid helps decide. Methylmalonic acid rises in B12 deficiency but not in folate deficiency, while homocysteine rises in both ⁴.
4. 4If B12 deficiency is confirmedAn intrinsic factor antibody test checks for pernicious anemia (the autoimmune cause, where the stomach stops making the factor that absorbs B12). If it is positive, B12 usually has to be given by injection.
5. 5If nothing explains itEspecially over 60, or with other low counts, a hematology referral and sometimes a bone marrow exam look for myelodysplasia.

Is a high MCV serious?

Most mildly raised MCVs with a normal hemoglobin are benign or reversible, and many trace to alcohol or a medicine. B12 deficiency carries the most risk, because it can cause numbness, tingling, balance and memory problems even without anemia or a high MCV ⁵, and that nerve damage can become permanent if it runs untreated; the B12 deficiency guide covers it in full. Folic acid taken alone by someone short of B12 can correct the blood count while nerve damage continues ⁴, so B12 needs checking before folate is started.

A high MCV on its own does not mean cancer. Myelodysplastic syndrome is the main marrow cause, and it usually shows up as macrocytosis plus low platelets or white cells in an older adult, rather than a lone high MCV ¹.

How MCV comes back down

MCV falls when the cause is treated, and it falls slowly. Red cells live about 120 days ², so the large cells already in circulation have to age out as new, normal-sized ones replace them. That is why MCV can take up to about four months to settle fully.

• B12 deficiency: replaced by tablets or injections depending on the cause; pernicious anemia and absorption problems usually need injections. See vitamin B12.
• Folate deficiency: replaced once B12 deficiency is excluded or treated. See folate.
• Alcohol: MCV corrects with abstinence ¹, with the last large cells clearing over the few months it takes red cells to turn over.
• Hypothyroidism: MCV settles as thyroid treatment takes effect.
• Medicines: some drug-related macrocytosis is expected and harmless, as with hydroxyurea. Do not stop a prescribed medicine because of an MCV result; review it with the prescriber.

A typical retest point is about 8 to 12 weeks after the cause is addressed, agreed with your doctor, and on the same lab if possible so the reference range matches. For anemia that comes with a high MCV, the anemia page covers the wider picture, and the blood count topic lists every marker on the report.

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