Blood Tests: Understanding Their Use

The blood test is a routine test that is performed without suspicion of disease on many occasions. In other cases, it allows detecting a pathology in its initial stages.
Blood Tests: Understanding Their Use
Samuel Antonio Sánchez Amador

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 15 March, 2021

Blood tests, as their name suggests, is a routine test that’s performed to count the number and types of cells that are present in this fluid. In addition to the presence of red blood cells, leukocytes, and platelets, the concentration of certain substances of medical interest is also quantified.

Experts recommend that people have at least one blood test a year, in order to determine the levels of cholesterol, triglycerides, urea, glucose, and, in addition, to perform a complete blood count. This last study refers to the quantity and variations of blood elements (circulating cells).

Mismatches in a blood test don’t always have to imply an illness, but they’re sometimes indicative of conditions such as dyslipidemia, anemia, kidney failure, and even the presence of some types of cancer. If you’d like to know all about this common and essential lab test, read on.

The importance of blood

Blood is a connective tissue of a fluid nature that circulates through veins, capillaries, and arteries, in order to transport oxygen and nutrients to each and every one of the cells, collecting waste substances along the way.

It’s estimated that an adult human being has about 5 liters (nearly 9 pints) of circulating blood inside them and that, to mobilize it correctly, the heart must beat between 60 and 100 times per minute.

Understanding the life of living beings without blood is impossible, since this liquid allows the nutrition and metabolism of cell bodies. According to the Roche Patients medical portal, the general composition of blood is as follows:

  • Blood plasma represents 55% of its total volume. It’s a fluid that’s denser than water, with a translucent yellowish color and a salty taste. In addition to transporting formed elements to circulating cells, it also serves as a vehicle for nutrients and metabolic waste substances.
  • Within the blood plasma itself, 90% corresponds to water and 10% to biological substances, such as proteins. Plasma proteins are very important, as they allow the maintenance of the body’s oncotic pressure. Albumin is the most relevant, representing 54% of plasma proteins.
  • The formed elements make up the other 45% of the blood, and can be measured on the blood count. The dominant cells here are red blood cells, as there are 4.35 to 5.65 million red blood cells per microliter of blood in humans, with a ratio of 1000: 1 when compared to white blood cells.

The functions of blood

Despite being fluid in nature, blood performs immune, transport, and storage functions, among many other things. According to the Argentine Ministry of Health, some of the most relevant are the following:

  • Participating in the defense against infections, as it carries leukocytes to the affected tissues.
  • Carrying both oxygen from the lungs and nutrients from the digestive system to the cells that need them.
  • Carrying various waste substances to the kidneys, and carbon dioxide to the lungs.
  • Participating in body thermoregulation.
  • Carrying hormones, enzymes, and many other regulatory biological molecules.
  • Participating in coagulation, since platelets travel inside it.

Such is the importance of this tissue that, in events such as cerebrovascular accidents – in which the blood supply to brain tissue is cut off – it takes a few minutes for the affected cells to begin to die.

Blood tests

As we’ve already explained the composition and function of blood, it’s time to answer the following questions: Who should have a blood test? How are they done? What do the results mean? In the following lines, we’ll answer each and every one of the questions raised here.

Who should have a blood test?

The blood test must be performed by professionals in clinical analysis.
In hospitals and clinics, the sample collection is usually carried out by the nursing staff, and the clinical analysis specialist is in charge of reviewing the sample.

As indicated by the Kidshealth portal, a blood test is usually part of a routine process, but it can also be requested as a screening test, or because the patient is not feeling well. In general, it is indicated for the following:

  • Monitoring the general health of the patient: As we have said before, it is always advisable to do a blood test every year.
  • Diagnosing a disease: An analysis may be suggested when the patient feels fatigue, malaise, fever, inflammation, bruising, or bleeding anywhere on the body. The examination isn’t usually the definitive diagnostic test, but it does allow the specialist to be guided.
  • Controlling an already-existent illness.
  • Monitoring a medical treatment: Blood tests are essential in patients with treatments that can alter the cell count in the bloodstream.

So, if a doctor asks you for a blood test, don’t be alarmed. The most normal thing is for us all to have one on an annual basis. And, if you haven’t got any symptoms, then doctors aren’t necessarily looking for anything in particular.

How are they done?

Most of us have had a blood test at some point: a needle, a quick prick, a cotton ball to prevent more blood from coming out, and home. Even so, the Líder Doctor portal gives us some advice to ensure that this blood test will show a true reflection of the patient’s current health status. Among them, we find the following:

  1. Maintain your eating habits and don’t change them in the days prior to taking the blood sample.
  2. Have the blood test first thing in the morning, always on an empty stomach. Of course, you can drink water and take the medication you need for any other condition, unless the doctor tells you otherwise.
  3. Don’t smoke before having the test.
  4. If you have a tendency to get dizzy during the extraction, don’t be shy and tell the nurses. If so, you will likely be placed lying down. Don’t be afraid to tell them if you’re feeling any sort of discomfort.
  5. Once the extraction is finished, lightly press the area of the puncture for about 10 minutes with cotton. This will prevent the appearance of bruises.

In the blink of an eye, the test is done. Usually, the amount taken is 10 to 12 milliliters, so you will hardly even realize you’re missing any! Once the required volume has been collected, the blood is sent to the laboratory for analysis.

What do the blood tests evaluate?

It’s time to get a bit technical now, because it’s time to reel off all the parameters that are taken into account when analyzing the samples from blood tests. We’ll begin by saying the following: the results are divided into two phases, the complete blood count and the biochemical portion. The Cultural Association of Diabetics of Cáceres helps us with this.

The blood count

As indicated by the Mayo Clinic, the hemogram measures the amount and variations of blood elements, that is, specialized circulating cells. Among them, we find the following:

Erythrocytes – red blood cells

Red blood cells are specialized cellular corpuscles, lacking nuclei and mitochondria, which carry oxygen thanks to hemoglobin. The mean for men is 5,000,000 and for women 4,500,000 per cubic millimeter. A lower than normal value will trigger some type of anemia, such as sickle cell disease.


This is a similar parameter, as it measures the percentage of red blood cells present in the blood. It’s a good indicator to calculate anemia.

Terms in acronyms

We’ll include three terms in a single category, as they all have something in common. These are as follows:

  • MCV (mean corpuscular volume): This refers to the mean individual volume of red blood cells, that is, their size. When there’s a lack of iron in the patient, this value tends to decrease.
  • HCM (mean corpuscular hemoglobin): The proportion of hemoglobin that each red blood cell contains. The MCHC is a similar value, as it relates the amount of hemoglobin in reference to the volume of the red cell.
  • ESR (erythrocyte sedimentation rate): This is a value that indicates the speed with which the red blood cells aggregate and settle, that is, how long it takes them to separate from the plasma.

Leukocyte formula

We’re not going to dwell too much on numerical clusters, as it’s enough to know that the leukocyte formula quantifies the proportion of white blood cells present in the blood. This includes neutrophils, eosinophils, basophils, monocytes, and lymphocytes.

A mismatch in the leukocyte formula can signify an allergy or inflammatory processes, among others.


Platelets are the cells in charge of plugging wounds, initiating blood clot formation, and preventing the loss of this tissue due to hemorrhage. The normal count is between 150,000 and 400,000 units per cubic millimeter. Its deficiency can show illnesses such as immune thrombocytopenia.

Platelet measurement is an essential part of the blood test.
Platelet measurement is very helpful in determining hemostasis problems.

The biochemical portion

Typical circulating blood cell bodies aren’t measured in this part of the test. Some of the biological compounds that are quantified are the following:

  • Glucose: An excess of glucose in the blood is known as hyperglycemia, a typical condition of diabetes. When insufficient or defective insulin is produced, glucose remains in the bloodstream instead of being used by cells.
  • Urea: Urea is excreted with urine, which is produced in the kidneys. An excess of urea in the blood can mean kidney failure in the patient. Other indicators of kidney function are uric acid and creatinine.
  • Cholesterol: The blood level of cholesterol. What we all know as ” high cholesterol ” is the excessive presence of this lipid in the blood, which can be deposited in the blood vessels and generate atheroma plaques.
  • Transaminases: Transaminases are a set of enzymes that are involved in metabolism and in the production of various amino acids. A high value in this parameter usually indicates poor liver function, among other pathologies.

As you can see, the count of cell bodies is as important as other plasma substances.

Many of these values can vary depending on the patient without this being a problem but, sometimes, these imbalances can be the first indication of an underlying pathology.


As you can see, blood tests measure a lot of things. From the proportion of circulating red blood cells to the presence of cholesterol, there’s a wide range of biological parameters that provide very relevant information to medical specialists about the general state of health of the patient.

For all these reasons, you don’t have to feel bad to get a blood test. This is a free, quick, and almost painless test, but it can detect many physiological problems before they become too serious. Make blood tests routine – you’ll appreciate them in the future!

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