Doctor prepares to draw patient's blood

Our blood contains a variety of substances as well as many different types of blood cells. As such, it serves as a biomarker for numerous health metrics. The National Cancer Institute (NCI) defines a blood test as a test performed on a blood sample to measure the presence of certain substances or types of blood cells. The NCI further explains that such tests typically look for signs of disease, antibodies, or tumor markers.

A book from the National Center for Biotechnology Information explains that there are three main types of blood cells: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes), in addition to blood plasma (the substance through which the cells travel). Plasma is the most abundant substance in the blood, comprising mostly water. Red blood cells carry oxygen to every part of the body. Platelets are cell fragments that help the blood to clot, a crucial function in wound healing. Meanwhile, there are many types of white blood cells, each with an important function for the immune system. For instance, antibodies are made from B cells, a type of white blood cell (per the Encyclopedia Britannica). Antibodies are highly specialized proteins, each designed to recognize and deactivate a specific antigen, and they are also found in our blood.

A complete blood count test measures how many of each of these types of blood cells you have in your blood, as well as some more specific components of blood (per the Mayo Clinic).

Blood tests can measure many different things

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According to the NCI, blood tests are usually used to measure various disease agents, such as antibodies or tumor markers. The NCI also says blood tests can be used to see how well a treatment is working. This is generally the purpose of a follow-up test, after an initial test leads to some type of prescription. Depending on what’s being treated, it’s important to get a follow-up test to see whether or not the prescribed treatment is having the intended effect.

But these are far from the only uses of blood tests. For instance, Healthline notes that many common blood tests measure various nutrients (such as sodium, glucose, protein, or cholesterol), each of which can help to diagnose different health conditions. There are also more complete nutrient tests that can reveal blood levels of essential micronutrients, as well as sugars, proteins, and lipids (fat).

Other blood tests can measure hormone levels, various enzymes, specific antibodies that may indicate the presence of an infection, and a wide variety of other compounds that can help give a picture of our current state of health. That said, they can’t tell us everything, and results can sometimes be ambiguous. When you receive blood test results, it’s important to speak with a healthcare provider to help you interpret them and understand what they might mean.

Routine blood work usually includes multiple tests

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If you see a doctor for routine blood work, this will most likely include a complete blood count, a basic metabolic panel, a thyroid panel, and a nutrient test of any or all nutrients (per Healthline). Common nutrients that get measured in a routine blood test include B vitamins, iron, calcium, sodium, and potassium.

A complete blood count measures the levels of all the different types of blood cells in your blood, which can help to show signs of cancer or immune problems, certain nutritional deficiencies, blood clotting problems, or signs of infection. A basic metabolic panel looks at certain key nutrients involved in regulating the metabolism and requires a person to have fasted for 8-12 hours prior to the test, also known as a fasting blood test. Metabolic panels are used most often to look for signs of diabetes, a condition that affects more than 10% of Americans (via the CDC). A thyroid panel can be used to measure thyroid function and certain types of hormones. Lastly, nutrient panels can measure many different types of nutrients for various purposes. For instance, a lipid panel will measure your cholesterol, which can be used as a sign of heart disease. Lipid panels and some other nutrient tests also require you to fast beforehand.

You should get a blood test if it’s been over a year since your last one

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How do you know when it’s time to get a blood test? Sometimes, you’ll want to get a blood test when you experience certain symptoms, or when you’re following a specific treatment plan and need to track your progress. But you don’t actually need a specific reason to get a blood test. As Healthline explains, it’s recommended that different age groups get routine blood work done at least every couple of years, but older age groups may want to get blood tests more frequently. Healthline suggests those under 40 can get blood work done every five years, while those over 40 may want to increase the frequency to every 1-3 years.

Other sources recommend even more frequent blood work. According to LaSante Health Center, getting blood work done every year, even when no symptoms are present, may sometimes be the norm. Many health centers and doctors will suggest annual blood work to be safe, partly because health conditions don’t always have symptoms. Getting tested every year helps catch potential conditions before they become serious.

Blood tests are invasive, requiring the use of a needle to puncture one of your veins, but the wound is very minimal and tends to heal quickly. Some people are understandably squeamish at the thought of having one of their veins punctured, but blood tests are perfectly safe and can be done multiple times a year if necessary.

You should get a blood test if you’re constantly feeling hungry or thirsty

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If you’re experiencing excessive hunger or thirst, this could be a sign of diabetes (via the CDC). A blood glucose test, or fasting blood sugar, can diagnose diabetes. Diabetes affects more than 10% of the U.S. population (via the CDC), and as such, blood glucose tests are among the most commonly used blood tests.

That said, excessive hunger and thirst could also be a sign of other conditions, many of which can also be measured with blood tests. As Healthline explains, excessive hunger (polyphagia) could be a sign of thyroid problems, nutrient deficiencies, or hypoglycemia (low blood sugar) that’s unrelated to diabetes, among other things. Additionally, excessive thirst is also a symptom of anemia, hypercalcemia (too much calcium), or other nutrition problems. All of these problems can potentially be diagnosed with the right blood test. When requesting blood work, it’s important to list all of your current symptoms and make sure you’re getting every relevant blood test.

You should get a blood test if you experience palpitations or chest pain

Man with chest pain

Heart disease is the leading cause of death in the United States, contributing to 25% of all deaths (according to the CDC). It may have some noticeable symptoms, including angina (chest pain) or heart palpitations (via the Mayo Clinic). Other symptoms include shortness of breath, lightheadedness or dizziness, or other types of pain, particularly neck, jaw, upper abdomen, or back pain. Additionally, numbness or weakness in your arms or legs can be a sign of narrowed blood vessels, a complication of heart disease. Any of these symptoms is sufficient reason to get your blood tested, though not every case of heart disease will have noticeable symptoms (which is why routine blood work is a good idea).

A number of different blood tests can help detect heart disease. A lipid panel measures cholesterol and triglycerides in the blood, while other tests can detect compounds that may be present after a cardiac event (via the Australia Department of Health). All of these together can help diagnose heart disease, though your doctor will likely want to follow up with a variety of other types of medical tests to confirm the diagnosis and determine how far along the condition has progressed.

You should get a blood test if you are at risk or have a history of high cholesterol

Doctor speaking with woman

According to the Mayo Clinic, high cholesterol typically has no symptoms, and it can only be diagnosed with blood work. Cholesterol is measured with a blood test known as a lipid panel, which will also measure triglycerides. High cholesterol can be an early sign of heart disease, which is responsible for 25% of U.S. deaths (via the CDC).

Because high cholesterol has no symptoms, it’s especially important for those who have a family history of high cholesterol or are otherwise at risk make sure to get tested about once a year. According to Johns Hopkins Medicine, high cholesterol can be hereditary, sometimes caused by a particular genetic mutation. If members of your family had heart attacks or other signs of heart disease earlier in life (before age 55), this could be a sign that you may have the genetic mutation that causes high cholesterol. For such cases, changing your diet may not be enough, and your doctor may prescribe cholesterol medication.

In other cases, high cholesterol can be the result of a poor diet or other lifestyle factors. For instance, the Mayo Clinic lists a diet high in saturated or trans fats as well as smoking, alcohol consumption, and a sedentary lifestyle to be additional risk factors for high cholesterol. It’s important for those who follow such habits to have routine blood work done to check cholesterol levels.

You should get a blood test if you have unexplained changes in weight

Person stepping on scale

Changes in weight that can’t be attributed to diet, exercise, or medication are another sign that it could be time for a blood test. As the Cleveland Clinic explains, unexplained weight change can be a symptom of thyroid problems or thyroid disease, which is actually a group of diseases affecting the thyroid. The thyroid is a gland responsible for creating and releasing a variety of hormones, many of which regulate your metabolism. Both weight loss and weight gain can be a sign of different thyroid problems: Hypothyroidism indicates an excessively slow metabolism and can cause weight gain, while hyperthyroidism suggests your metabolism is running too fast and can lead to weight loss.

The Cleveland Clinic goes on to suggest thyroid disorders are very common, affecting approximately 20 million people in the U.S. With this in mind, it’s not surprising that thyroid tests are one of the more common types of blood work. According to Healthline, a thyroid panel — which measures thyroid function and can help to diagnose thyroid disorders like hypothyroidism and hyperthyroidism — is typically part of routine blood work. Though thyroid disorders often have a variety of symptoms, they may not be noticeable in the early stages, but routine blood work can help you catch a problem before it starts giving you symptoms.

You should get a blood test if you have unexplained changes in mood

Woman experiencing mood swings

Rapid shifts in mood can be caused by a wide variety of conditions, according to Healthline. For instance, bipolar disorder and other mental health conditions can cause extreme moods and mood swings. These generally aren’t diagnosed with blood tests, and they also tend to follow predictable patterns. But when you experience sudden changes in mood that can’t be attributed to a mood disorder — and even if you have been diagnosed with a mood disorder — unfamiliar changes in mood could be a sign of another health condition.

Hormone changes, thyroid disorders, conditions affecting the lungs or cardiovascular system, and central nervous system problems can all cause sudden changes in mood. Additionally, diet can play a central role in mood, and changes in mood can also result from dietary changes or nutrient deficiencies. All of these potential causes would be revealed with routine blood work, which typically measures hormones (especially hormones relating to the thyroid), nutrient levels, and important cardiac biomarkers among other things (via Healthline).

You should get a blood test if you’re experiencing severe hair loss

Woman concerned about hair loss

Per Healthline, some hair loss (also known as alopecia) is common with age in both men and women. Age-related hair loss can begin as early as the late teen years, and it’s very common by age 50 or 60. This type of gradual hair loss is genetic; it tends to run in families, and by itself, it’s not a symptom of any condition. However, when significant hair loss happens suddenly, this is likely a sign of an underlying health condition.

Healthline suggests hair loss can be a sign of nutrient deficiency, as can brittle hair. Biotin (vitamin B7), niacin (vitamin B3), iron, zinc, and fatty acids linoleic acid and alpha-linoleic acid are all crucial for hair growth, so deficiencies in these (and other) nutrients have the potential to cause hair loss. Additionally, according to the Cleveland Clinic, thyroid problems can also cause hair loss or dry, coarse hair.

Blood work can assess nutrient levels as well as thyroid function, via two different common blood tests. These tests are a good option for those concerned about hair loss who want to make sure they don’t have underlying health conditions.

You should get a blood test if you’re excessively fatigued

Tired man at computer yawning

If you’re feeling extremely tired, the first thing to ask yourself is whether or not you’ve been getting enough sleep. The CDC estimates that more than a third of Americans may be sleep deprived, falling short of the minimum of seven hours of sleep recommended by the American Academy of Sleep Medicine. In this case, the diagnosis and prescription are fairly straightforward: You’re tired because you haven’t slept enough, and you should probably try to get more sleep.

That said, excessive fatigue that persists even when you feel you’re getting plenty of sleep can be a sign of an underlying health condition. As Healthline explains, depression can contribute to fatigue, although they also point out that chronic fatigue syndrome is often misdiagnosed as depression. Chronic fatigue syndrome refers to excessive fatigue that can’t be attributed to any known condition. Both depression and chronic fatigue syndrome are typically diagnosed via psychological evaluation or survey, but other conditions that cause excess fatigue can be measured with blood tests.

As an example, chronic kidney disease can cause fatigue and weakness, according to the Mayo Clinic. Kidney function can be measured via blood test. Additionally, WebMD lists viral or bacterial infections, heart conditions, diabetes, thyroid problems, and cancer as other potential causes of unexplained fatigue. Various blood tests can be used to test for any and all of these conditions.

You should get a blood test if you are sexually active

Smiling couple sitting on couch

Another common type of blood tests measures sexually-transmitted diseases (STDs). The Mayo Clinic notes that STDs are common, and often don’t have any symptoms. As with other conditions that might not have symptoms, it’s a good idea to get routine blood work to test for these conditions.

When it comes to STDs, your habits will also determine the frequency and type of tests you need. The Mayo Clinic recommends getting tested every time you have a new sexual partner, or if your sexual partner has other sexual partners. This applies not just to intercourse but to all types of sexual activity, even when you use protection, which always has a slight chance of failure. Women who are pregnant or planning on becoming pregnant are also recommended to have STD tests.

Blood tests are sufficient to diagnose most STDs. Urine tests can also be used to screen for STDs, and some of these can be purchased over-the-counter and used at home. That said, home tests have a higher rate of false positives, meaning there’s a greater chance they’ll tell you that you have an STD you don’t really have. As such, home tests with positive results should always be followed up with additional tests to confirm the diagnosis, such as blood tests.

You should get a blood test if you think you have an infection

Woman with fever in bed

If you’re experiencing symptoms of an infection, or if you know you’ve been exposed to a specific type of infectious disease (e.g. via an infected person or traveling to a hot spot for an infectious disease), a blood test can help determine whether or not you have signs of an infection in your blood. As WebMD explains, an infection that is present in your blood is known as a systemic infection, and it can be measured with a blood culture test. Symptoms of systemic infection include fever, excess fatigue, nausea, rapid heart rate, or confusion, among others. However, it’s possible to have a systemic infection with no symptoms.

Blood culture tests add specific cultures to your blood sample (hence the name), as a way to determine whether or not certain bacteria or yeast are present. If they are, the culture will cause measurable growth in the bacteria or yeast within 24-72 hours. However, note that blood culture tests can’t show signs of viral infection.

For viral infection, antibody blood tests can show evidence of the infection. As the NCI suggests, antibody tests are among the most common types of blood tests. Antibody tests can show current or past signs of infection. For instance, the CDC says antibody tests can be used to determine whether or not a person has been exposed to the virus causing COVID-19, but they shouldn’t be relied on to diagnose current infection.

You should get a follow-up blood test if a recent blood test gave you abnormal results

Doctor goes over patient test results

When routine blood work raises potential issues (e.g., if your B12 was a little low, or your cholesterol was a little high, or you found evidence of some antibodies, etc.), your healthcare provider may prescribe some type of treatment and request a follow-up blood test to see how the treatment is working. Harvard Health recommends asking your healthcare provider whether or not you need to repeat a blood test that gave you abnormal results.

To be clear, what is "normal" actually depends somewhat on each individual. Ideally, your doctor has several years’ worth of blood test data from you to work with. Dr. Suzanne Salamon of Harvard Medical School gives an example of what this might look like in practice: When a person usually tests in the low normal range for some metric, and then one day they test in the high normal range, this may be cause for concern, even though their test is still technically reading as "normal." Dr. Salamon says this might spark a conversation about any recent diet or habit changes. In such cases, especially when you want to work on getting the metric back to your usual level, you may want to do a follow-up blood test to keep track of your progress.

Remember that other types of medical tests are important too

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As we’ve seen, blood tests can be an important part of routine health care. Healthline notes that there are a variety of blood tests included in routine blood work that can diagnose or act as evidence for a wide variety of conditions. What’s more, some of these conditions may not present symptoms, especially in the early stages. For this reason, scheduling routine blood work every year or couple of years is widely recommended, even when you’re not experiencing any unusual symptoms.

Keep in mind, however, that blood tests are not the only important medical tests that can diagnose conditions or give you a picture of the state of your health. For instance, urinalysis is another important type of medical test that uses your urine instead of your blood, making it less invasive. The Mayo Clinic says urinalysis can diagnose certain conditions such as kidney problems. Additionally, stool tests can be used to test for bacterial infection (via WebMD) and salivary tests can help to detect viral infection (via a study from the Journal of the American Dental Association), in addition to a variety of other conditions.

However, such tests are not usually done as part of a routine, but only when certain symptoms are present or other types of tests have had positive results. When it comes to keeping your health in check, routine blood work is a relatively easy way to stay on top of any potential health problems.