Oxygen and Blood: Knowing the Difference Between SpO2 Vs. Hemoglobin


No competition exists between oxygen saturation (SpO2) and hemoglobin. That’s because oxygen saturation refers to a measurement of oxygen-carrying hemoglobin in the bloodstream. Instead of pitting them against each other, as sodium and potassium sometimes compete, oxygen saturation describes the body’s hemoglobin. Let’s look at each in more detail and see how they fit together.


Oxygen Saturation Defined

The term oxygen saturation refers to a measurement of how much oxygen-carrying hemoglobin appears in the bloodstream relative to the hemoglobin that does not carry oxygen. To function properly, the human body needs oxygenated blood. In fact, humans need about 90% or more of their blood hemoglobin to contain oxygen. A reading of oxygen saturation reflects the fraction or ratio of oxygen-saturated hemoglobin relative to the total hemoglobin.


How to Measure Oxygen Saturation

Pulse oximetry uses a small device called a pulse oximeter to measure oxygen saturation. This provides healthcare professionals with data regarding the percentage of oxygen-loaded hemoglobin molecules in arterial blood. Typically used in a hospital environment, the pulse oximeter slips onto a finger or toe to measure oxygen saturation non-invasively through the skin.

Some smartwatches and fitness trackers measure oxygen saturation using a sensor on the back of the watch. It rests against the skin and takes a measurement in a similar manner to pulse oximeters. Although fine for fitness monitoring, these measurements made with consumer devices should not be used for medical decisions. If you received a diagnosis of a disease that requires monitoring of your SpO2 percentage, purchase a pulse oximeter.

Learn how to install SpO2 on your smartwatch.


Why Oxygen Saturation Matters

Oxygen saturation provides one of the vital signs communicating the health of a person’s blood. If the SpO2 reading falls below 90%, hypoxemia develops. The blue skin that provides the most visible sign of hypoxemia signals cyanosis has developed.

Without adding oxygen to the individual’s hemoglobin, hypoxemia can develop into hypoxia, a condition of low oxygen levels in bodily tissues. A lack of oxygen to the bodily tissues can cause heart damage, brain damage, or death.

Knowing the range of healthy oxygen saturation readings and your own saturation levels with your in-home pulse oximeter will help you plan the appropriate actions to take when your SpO2 reading falls below 90%. Note that you should not jump to conclusions when your oxygen saturation falls below normal levels. Seek out professional medical assistance.


Normal SpO2 Levels in the Human Body

When an individual breathes normally, their lungs send the oxygen they inhale throughout the bloodstream, and it binds to hemoglobin. Hemoglobin traverses the entire human body. Under normal conditions, expect a reading of 95% to 100% in a healthy individual under typical circumstances. 

Certain activities, such as dancing, working out, and playing sports, increase the human body’s need for oxygenated blood. When watching a football game on TV, you might observe a football player using an oxygen mask to regulate their breathing between plays. This helps raise the oxygen level in the player’s blood.

Certain diseases also affect the normal oxygen saturation levels of an individual. For example, chronic obstructive pulmonary disease (COPD) results in normal levels between 88% and 92%. Anything lower than 88% indicates a need to go to the hospital.

As oxygen saturation drops, a person becomes out of breath and lethargic. They may feel light-headed. Those signs tell the football player to head to the bench for the oxygen tank. If they measured their oxygen saturation as soon as they reached the bench, it would probably hover around 90%.

Now, let’s discuss hemoglobin, an important component in human blood.


Hemoglobin Defined

The term hemoglobin refers to a protein in the red blood cells. This protein carries oxygen to each of the human body’s organs and tissues. It also collects carbon dioxide present in the body’s organs and tissues and transports it back to the lungs.


Measuring Hemoglobin

Just as medical staff use a simple test for oxygen saturation, they also use a straightforward hemoglobin test that determines how much hemoglobin the individual has in their blood. A low hemoglobin measurement indicates a low red blood cell count, a sign of anemia. Various diseases cause anemia, ranging from simple chronic disorders to bleeding issues to vitamin deficiency.

Too high hemoglobin levels also indicate a problem. These range from dehydration to the blood disorder polycythemia vera. If you smoke or live at a high altitude, this also causes high hemoglobin levels.


Why Would You Have a Hemoglobin Test?

A doctor typically measures a patient’s hemoglobin during their annual checkup. It comprises one part of a complete blood count. The hemoglobin test screens for various disorders, like anemia.

If you experience fatigue, dizziness, shortness of breath, or weakness, your doctor may run a hemoglobin test. Because the symptoms also indicate the possibility of a lack of oxygen in the blood, the doctor will also check the individual’s oxygen saturation.

The doctor will order periodic hemoglobin tests to monitor a patient’s condition if they are diagnosed with a medical condition.


What Happens during a Hemoglobin Test?

Typically, a medical technologist, nurse, or doctor pricks the patient’s finger with a small needle. They then squeeze a tiny droplet of blood onto a slide. In some cases, the medical staff may need to take the blood from a vein using a special needle and blood-collection tube. To take a blood sample from an infant, the medical staff uses a needle prick on the heel of the foot.

Unlike pulse oximetry, which provides an immediate reading, the hemoglobin test requires analysis. The blood sample goes to a lab for processing. If the doctor has an in-house lab, the hemoglobin test processing typically takes only a few hours. The results may take a few days if the doctor needs to send the blood sample to an outside lab.

Since this test usually occurs during a typical annual physical exam, you don’t have to avoid eating or drinking before the test.


Normal Hemoglobin Levels in the Human Body

Similar to oxygen saturation, hemoglobin also has a typical healthy range. This varies depending on the age and gender of the individual. 

  • For an adult male, the healthy range falls between 13.2 grams and 16.6 grams per deciliter.
  • For an adult female, the healthy range falls between 11.6 grams and 15 grams per deciliter.

Unlike adults, the healthy ranges vary for children. Their gender and age influence their normal range.


Lower than Normal Hemoglobin

What happens if your results dip below 13.2 grams or 11.6 grams per deciliter, depending on your gender? Lower-than-normal results indicate a form of anemia. Each anemia type stems from different possible causes:

  • Bleeding
  • Blood and bone marrow cancers
  • Folate deficiency
  • Iron deficiency
  • Hypothyroidism
  • Kidney disease
  • Liver disease
  • Thalassemia
  • Vitamin B-12 deficiency

The result of this test, combined with results from other tests, helps doctors hone the list of potential diagnoses. It may lead to a new round of more complex tests.


Higher than Normal Hemoglobin

If your hemoglobin level tests higher than normal levels, that also points to various health issues:

  • Burns
  • Dehydration
  • Excessive vomiting
  • Extreme physical exercise
  • Lung disease
  • Living at a high altitude
  • Polycythemia vera
  • Smoking heavily

Similar to too-low readings, this may lead to a new round of medical tests that determine the reason for the too-high readings.


Terms Used Commonly in Discussing Hemoglobin

Let’s consider a few specific terms related to hemoglobin and what they mean.

  • Oxygenated hemoglobin occurs during respiration when oxygen molecules and heme groups of hemoglobin bind together in red blood cells. Oxygenated hemoglobin appears a bright crimson red.
  • Deoxygenated hemoglobin lacks oxygen and occurs during the tense state or T state of hemoglobin. Deoxygenated hemoglobin appears dark red.
  • Saturated hemoglobin describes hemoglobin that carries the maximum of four molecules of oxygen it can accommodate. Each hemoglobin molecule has four oxygen-binding sites. 
  • Unsaturated hemoglobin is hemoglobin without oxygen attached to its four Fe+2 atoms. It could have oxygen attached to no available molecule or up to three.
  • Hemoglobin dissociation curve is a sigmoidal graph depicting the relationship between the amount of oxygen attached to hemoglobin and the partial pressure of oxygen in the blood. 



It isn’t a matter of SpO2 versus hemoglobin because SpO2 describes a measurement related to hemoglobin. Both tests comprise a part of the typical annual exam. Oxygen saturation is a percentage of the amount of hemoglobin carrying oxygen in an individual’s blood. A hemoglobin test reveals the amount of hemoglobin in the individual’s bloodstream expressed in grams per deciliter.