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9 Things You Didn’t Know You Could Learn from a Single Blood Test

You'll be shocked at all the medical conditions your doctor can diagnose from a teaspoon of your blood.

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The powerhouse test

In the average blood test, 3 to 10 milliliters (1 to 3 teaspoons) of blood is taken from the body through a needle. That’s not much—but doctors can learn a lot from that small amount. A complete blood count (CBC), commonly ordered at an annual checkup, measures your red blood cells, white blood cells, and platelets. It helps evaluate your overall health and can also detect a wide range of health problems, from anemia to leukemia. Your doctor may also check more specifically for certain viruses, hormones, or other substances; following are some of the results you may not know about.

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You’re pregnant (before you even missed a period)

A urine test can detect the pregnancy hormone (hCG) one or two days after your period should have started. “But a blood test can detect hCG at a much lower level than a urine test, so you’ll know a few days before that,” says Deborah Nunziato-Ghobashy, MD, family medicine physician at Scarsdale Medical Group.

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Your fertility

“There are certain things you can tell from maternal bloodwork,” says Dr. Nunziato-Ghobashy. “You can tell the age of the mother’s egg cells, for example, which clues you into whether it may be a little more difficult to become pregnant at that point.” Don’t miss these 9 secrets your blood type reveals about you.

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Your biological age

Your date of birth tells your chronological age, but several genetic and lifestyle factors can lower or increase your biological age—how old your body seems compared to other people your age. A very healthy 60-year-old who eats a nutritious diet, exercises, and doesn’t smoke could have the biological age of a 50-year-old. “The way your cells appear can be checked through blood work,” says Dr. Nunziato-Ghobashy. “And if you seem older than you really are, you may be able to alter that through lifestyle.”

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You’re depressed or anxious

A blood test alone can’t diagnose mental health problems, but in combination with a clinical evaluation, it can offer clues. “There are specific mood neurotransmitters, such as serotonin, that can be checked in the blood,” says Dr. Nunziato-Ghobashy. (Serotonin is found in blood platelets and in the digestive tract.) People with low levels may suffer from depression or anxiety.

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You’re at higher risk for dementia

Whether a person gets Alzheimer’s disease or another form of dementia can depend on numerous risk factors—but a blood test can reveal the presence of certain chemicals that raise a person’s risk. Specifically, a protein called amyloid beta can be detected up to 20 years before the onset of dementia. A positive test doesn’t always mean you’ll get dementia, says Dr. Nunziato-Ghobashy. “It just means you are predisposed.” Find out the lifesaving medical tests you might be skipping—but shouldn’t.

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You’ve had a concussion

Doctors can use a neurological exam, brain imaging, and close observation to determine whether someone has suffered a concussion, according to the Mayo Clinic. But a blood test can help: “It may show an elevation in serum electrolytes, which can be an indicator,” says Dr. Nunziato-Ghobashy.

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Your kidneys aren’t working right

After a taxing workout, your muscles release creatinine—a waste product that’s filtered from your blood by your kidneys. If creatinine levels build up—greater than 1.2 for women and greater than 1.4 for men—it can indicate a problem with your kidneys, according to the National Kidney Foundation. So can an elevated level of blood urea nitrogen (BUN) and a Glomerular Filtration Rate (GFR) that is too low. All three can be measured in a blood test, which can also clue you into whether the cause is external (such as dehydration) or internal (such as a kidney stone). Find out more body changes that can signal a health issue.

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Your diabetes treatment is on track

The A1C blood test gives your doctor a sense of your average daily blood sugar levels and it can help diagnose diabetes: A level between 4 and 5.6 is normal, 5.7-6.4 indicates prediabetes (or a high risk of developing diabetes), and a level of 6.5 or higher indicates full-blown diabetes. But what’s less well known is that the same test can also be useful once the disease is diagnosed. “It measures your blood sugar average over the previous three months,” says Dr. Nunziato-Ghobashy. “It’s a more reliable number than a blood sugar reading on any given day, so I can actually tell how well their treatment is working.”

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You’re getting too much of a vitamin

It’s important to know whether you have a vitamin deficiency. But a vitamin overdose, which can also be detected with a blood test, can be dangerous as well. “Most vitamins are water-soluble, which means they’ll just pass through your system if you take too much,” says Dr. Nunziato-Ghobashy. “But vitamins A, D, E, and K are fat soluble, so they stick around in your system and you can overdose on them.” This condition is called hypervitaminosis and can create problems ranging from nausea to liver damage. Next, don’t miss the silent signs your body might be in trouble.