The ABO and RhD blood groups are two of the most important systems for classifying human blood. They are determined by the presence or absence of specific antigens on the surface of red blood cells.
The ABO Blood Group System
The ABO blood group system is based on the presence or absence of two antigens, A and B, on the surface of red blood cells. There are four possible ABO blood groups: A, B, AB, and O.
- People with type A blood have the A antigen on their red blood cells.
- People with type B blood have the B antigen on their red blood cells.
- People with type AB blood have both the A and B antigens on their red blood cells.
- People with type O blood have neither the A nor the B antigen on their red blood cells.
The RhD Blood Group System
The RhD blood group system is based on the presence or absence of the RhD antigen on the surface of red blood cells. There are two possible RhD blood types: RhD positive and RhD negative.
- People with RhD positive blood have the RhD antigen on their red blood cells.
- People with RhD negative blood do not have the RhD antigen on their red blood cells.
Blood Transfusion Compatibility
ABO and RhD blood group compatibility is important for blood transfusions. If a person receives a transfusion of blood that is incompatible with their ABO or RhD blood type, their immune system will attack the donor red blood cells. This can lead to a serious and potentially life-threatening reaction called a transfusion reaction.
- People with type A blood can safely receive blood from people with type A and type O blood.
- People with type B blood can safely receive blood from people with type B and type O blood.
- People with type AB blood can safely receive blood from people with all four blood types (A, B, AB, and O).
- People with type O blood can only safely receive blood from people with type O blood.
- People with RhD positive blood can safely receive blood from people with both RhD positive and RhD negative blood.
- People with RhD negative blood can only safely receive blood from people with RhD negative blood.
ABO and RhD blood grouping, also known as blood typing, is a crucial laboratory procedure that determines an individual’s blood group based on the presence or absence of specific antigens on the surface of their red blood cells. These antigens play a pivotal role in ensuring compatibility for blood transfusions and preventing hemolytic disease of the newborn (HDN). The tube method is a widely used technique for ABO and RhD blood grouping, offering simplicity, accuracy, and cost-effectiveness. Further investigations of the Rh system can be done using the Rh phenotyping protocol.
Principle of ABO and RhD Blood Group Testing
In the tube method, both serum and red blood cells are utilized to identify the ABO and RhD blood group of an individual. Serum, the liquid portion of blood remaining after clotting, contains antibodies that react with specific antigens on red blood cells. Red blood cells possess antigens that are either present or absent, determining the individual’s blood group. The principle of ABO and RhD blood group testing involves two distinct processes: forward grouping and reverse grouping.
Forward grouping, also known as cell grouping, directly identifies the antigens present on an individual’s red blood cells. It utilizes antisera, which contain antibodies specifically directed against A and B antigens. These antisera are added to a suspension of the patient’s red blood cells. If the patient’s red blood cells possess the corresponding antigen (A or B), they will agglutinate or clump together due to the antigen-antibody reaction.
Reverse grouping, also known as serum grouping, identifies the antibodies present in an individual’s serum, which are indicative of the antigens they lack on their red blood cells. It utilizes red blood cells of known A, B and O blood groups. The patient’s serum is added to each tube, and agglutination of the corresponding red blood cells indicates the presence of antibodies against that antigen (anti-A or anti-B).
In addition to ABO blood group determination, reverse grouping also serves as a quality control measure for forward grouping and helps identify individuals with irregular antibodies that may cause transfusion reactions.
- Polypropylene/glass tubes 10 mm x 75 mm
- Refrigerated centrifuge
- Pasteur pipettes
- Commercially prepared anti-A, -B, -A,B and -D sera
- Commercially prepared known A, B and O cells
- Patient’s EDTA whole blood
- Normal saline
Preparation of 2 – 5% red cell suspension and plasma
- Centrifuge patient’s whole blood at 1000 – 2000 x g for 10 minutes using refrigerated centrifuge.
- Label two tubes with the patient/donor’s name and ID, designating one for red cell suspension and the other for plasma.
- Use a Pasteur pipette to carefully extract the serum/plasma, avoiding touching the red blood cells layer. Transfer the extracted serum/plasma into the appropriately labeled plasma tube.
- Carefully insert the tip of the Pasteur pipette into the center of the red cell layer and aspirate a small quantity of red cells. Dispense one drop the aspirated red cells into the appropriately labeled tube for red cell suspension.
- Add saline solution to the tube containing the red cells until it is approximately three-quarters full (approximately 0.5 – 1 mL of saline). Thoroughly resuspend the red cells.
* For quality control purposes, visually compare the color of the prepared red cell suspension to that of a 3% commercial red cell suspension. Adjust the suspension strength if necessary. Alternatively, you can place one drop of the prepared red cell suspension and one drop of a commercial red cell suspension into separate test tubes, centrifuge the tubes, and compare the size of the resulting red cell buttons.
**Red cell suspensions should be used for testing on the day of preparation for more accurate results.
Forward Grouping (testing the patient’s red cells)
- Label four tubes with the patient/donor’s name and ID, assigning each tube a corresponding reagent label: anti-A, anti-B, anti-A,B, and anti-D.
- Dispense a drop of commercial anti-A serum into the tube labeled anti-A. Repeat this process for anti-B, anti-A,B, and anti-D, adding each serum to its respective labeled tube.
- Carefully add one drop of well-mixed prepared red cell suspension to each tube of the antiserum.
- Thoroughly mix the reagents and red cell suspension by gently flicking the base of each tube.
- Centrifuge the tubes according to the reagent manufacturer’s instructions. Usually, centrifugation is performed at 1500 rpm for 15-20 seconds using a serofuge.
- Gently resuspend the cell button in each tube and examine for agglutination.
- Interpret and record the test results. Compare the test results for the red cell suspension with those obtained from reverse grouping.
Reverse Grouping (testing the patient’s plasma)
- Label three tubes with the patient/donor’s name and ID, assigning each tube a corresponding cell type label: A cells, B cells, and O cells.
- Dispense a drop of commercially prepared known type A red cells into the tube labeled A cells. Repeat this process for known type B and type O red cells, adding each cell type to its respective labeled tube.
- Carefully add one drop of the patient’s plasma to each tube containing the corresponding known red cells.
- Thoroughly mix the contents of each tube by gently flicking.
- Centrifuge the tubes according to the reagent manufacturer’s instructions. Usually, centrifugation is performed at 1500 rpm (503 g) for 15-20 seconds using a serofuge.
- Examine the plasma overlying the cell buttons for any signs of hemolysis.
- Gently resuspend the cell buttons in each tube and examine for agglutination.
- Interpret and record the test results. Compare the test results for the plasma with those obtained from forward grouping.
Positive results in forward grouping and reverse grouping are indicated by agglutination of tested red cells and either hemolysis or agglutination in serum, respectively. A negative result is observed when there is a clear supernatant after resuspension in the red cell agglutination test.
Assessment of red cell agglutination test grading
|4+ / Complete (C)
|Macroscopically visible cell button with a clear supernatant.
|Macroscopically visible large clumps of cell button with a clear supernatant.
|Macroscopically visible small clumps of cell button with a clear supernatant.
|Just macroscopically visible fine granular clumps of the cell button and the supernatant is turbid and reddish.
|or weak (w)
|Only microscopically visible fine granules of the cell button and the supernatant is turbid.
|No agglutination seen. The supernatant is clear and reddish in color.
|MF (mixed field)
|A mixture of agglutinated and unagglutinated red cells seen.
|Complete hemolysis of the patient sample. The supernatant is grossly red with no evidence of red cells.
In forward grouping, four distinct tubes are prepared:
- Tube 1 contains anti-A serum and patient’s red blood cells.
- Tube 2 contains anti-B serum and patient’s red blood cells.
- Tube 3 contains anti-A,B serum and patient’s red blood cells.
- Tube 4 contains anti-D serum and patient’s red blood cells.
Based on the agglutination patterns observed in these tubes, the ABO blood group is determined:
- Agglutination in tubes 1 and 3 (anti-A serum) indicates type A blood.
- Agglutination in tubes 2 and 3 (anti-B serum) indicates type B blood.
- Agglutination in tubes 1, 2 and 3 indicates type AB blood.
- Absence of agglutination in tubes 1, 2 and 3 indicates type O blood.
- Agglutination in tube 4 indicates RhD positive blood.
- Absence of agglutination in tube 4 indicates RhD negative blood.
In reverse grouping, based on the agglutination patterns observed in the tubes, the RhD blood group is determined:
- Agglutination in tube 1 indicates the presence of anti-A antibodies, corresponding to type B or type O blood.
- Agglutination in tube 2 indicates the presence of anti-B antibodies, corresponding to type A or type O blood.
- Absence of agglutination in both tubes indicates the absence of anti-A and anti-B antibodies, corresponding to type AB blood.
- Agglutination in both tubes indicates the presence of anti-A and anti-B antibodies, corresponding to type O blood.
- Agglutination in tube 3 could indicate presence of alloantibodies.
While accurate and highly reliable, the tube method of ABO grouping can sometimes have false positive reactions. These can lead to incorrect blood typing, potentially posing risks in blood transfusions or other blood-related procedures. Here are some common causes of false positives:
- Patient misidentification: Mistakes in labeling or identifying patients can lead to testing the wrong blood sample.
- Improper sample collection: Contaminated samples due to improper collection technique or storage can interfere with testing.
- Incorrect sample type: Using the wrong type of anticoagulant tube or collecting insufficient blood volume can impact results.
- Improper centrifugation: Insufficient or excessive centrifugation can impact the visibility of agglutination reactions, leading to misinterpretation.
- Incorrect incubation: Deviation from recommended temperature or timeframes can affect how antibodies and antigens interact, causing false positives or negatives.
- Contamination: Contamination of reagents, glassware, or the sample itself with other blood products or even microorganisms can trigger non-specific agglutination.
- Pipetting errors: Using the wrong volume of antiserum or blood sample can distort the antigen-antibody ratio, impacting reaction sensitivity.
- Transcription errors: Mistakes in recording or transcribing test results can lead to miscommunication and errors in patient care.
- Interpretation errors: Improper interpretation of agglutination reactions, especially faint or borderline reactions, can lead to incorrect blood typing.
- Communication errors: Failure to effectively communicate results to healthcare providers or misinterpreting their requests can lead to errors in blood transfusions or other procedures.
- Weak antigens or antibodies: Weak antigens might not cause visible agglutination in forward grouping, but be detected by stronger antigens in reverse grouping, leading to misinterpretation. Similarly, weak antibodies might produce faint clumping, appearing positive.
- Rouleaux formation: This non-specific stacking of red blood cells can mimic agglutination, especially in low protein environments like saline. Careful microscopic examination is crucial to differentiate.
- Polyagglutination: Some rare antisera can react with a wider range of red blood cells, causing false positives regardless of the ABO type.
- Cold agglutinins: These are antibodies that activate at cooler temperatures, potentially causing nonspecific agglutination, especially in blood samples collected in cold environments.
- Recent immunizations or transfusions: Exposure to foreign antigens can trigger antibody production, leading to unexpected antibodies detectable in reverse grouping, potentially causing false positives.
- Certain medications: Drugs like some antibiotics or intravenous immunoglobulins can interfere with red blood cell agglutination reactions, impacting test results.
- Underlying medical conditions: Autoimmune diseases or certain infections can alter antigen or antibody expression, potentially contributing to false positives.
Prevention and Management
- Strict adherence to standardized protocols minimizes technical errors.
- Experienced technicians can better recognize and troubleshoot false positives.
- Quality control measures for reagents and equipment ensure accuracy.
- Repeating tests with fresh samples or different antisera can confirm or refute questionable results.
- Consulting specialists in transfusion medicine or immunology might be necessary for complex cases.
Frequently Asked Questions (FAQs)
What are the five steps in the blood typing procedure?
The five main steps in the blood typing procedure using the tube method are:
1. Sample Preparation
2. Forward Grouping
3. Observation and Recording
4. Reverse Grouping
5. Blood Type Determination
How do antigens and antibodies interact in ABO blood group typing?
In ABO blood typing, antigens and antibodies play a key role in determining your blood type.
Antigens: These are molecules present on the surface of the red blood cells (RBCs). In the ABO system, there are two main antigens: A and B. Individuals with type A blood have A antigens, those with type B have B antigens, those with type AB have both A and B, and those with type O have neither A nor B.
Antibodies: These are proteins produced by your immune system in response to foreign substances. In ABO typing, antibodies are present in your plasma and can react with specific antigens they recognize. For instance, someone with type A blood has anti-B antibodies, while someone with type B has anti-A antibodies. Type O individuals have both anti-A and anti-B antibodies, and type AB individuals have neither.
Agglutination Reaction: This is the key phenomenon observed in ABO blood typing. When an antigen on an RBC encounters its corresponding antibody in the plasma, they bind together, causing the RBCs to clump together in a visible mass called an agglutinate. This reaction occurs because the antigen and antibody have specific binding sites that fit together like a lock and key.
How it works in ABO typing:
- Forward grouping: RBCs are tested with antisera containing known antibodies (anti-A, anti-B, and anti-Rh). If an antigen on an RBC encounters its corresponding antibody in the antiserum, agglutination occurs, indicating a positive reaction. The absence of agglutination indicates the antigen is not present on the RBC.
- Reverse grouping: Plasma is tested with known RBCs (type A and B). If an antibody in the plasma encounters its corresponding antigen on the RBC, agglutination occurs, indicating a positive reaction. The absence of agglutination indicates the antibody is not present in the plasma.
Importance of agglutination:
- By observing agglutination patterns in both forward and reverse grouping, the specific antigen and antibody profile can be determined, ultimately revealing the individual’s blood type.
- This information is crucial for safe blood transfusions, ensuring compatibility between donor and recipient blood. Incompatible blood types can cause severe reactions.
What is the significance of performing both forward and reverse grouping in ABO blood typing?
Performing both forward and reverse grouping in ABO blood typing is crucial for ensuring accurate and reliable determination of an individual’s blood type. Here’s why each step is important and how they complement each other:
- Identifies antigens on red blood cells (RBCs): RBCs are tested with antisera containing known antibodies (anti-A, anti-B, and anti-Rh). If an antigen on an RBC encounters its corresponding antibody, agglutination occurs, confirming the presence of that antigen.
- Direct testing: This confirms what antigens are present on the RBC surface, which directly determines the blood type.
- Detects antibodies present in the plasma: Plasma is tested with known RBCs (type A and B). If an antibody in the plasma encounters its corresponding antigen on the RBC, agglutination occurs, indicating the presence of that antibody.
- Indirect testing: This confirms what antibodies are present in the plasma, which should logically correspond to the missing antigens on the RBCs for compatibility.
Significance of Both
- Verification: Comparing the results of both methods provides internal verification. Compatibility between forward and reverse grouping strengthens the accuracy of the blood type determination.
- Identifying Discrepancies: Inconsistent results between forward and reverse grouping (e.g., agglutination in both forward and reverse) can indicate potential issues like technical errors, weak antigens/antibodies, or rare blood types. This alerts the lab to investigate further before confirming the blood type.
- Ensuring Safety: Accurate blood type identification is crucial for safe blood transfusions. Matching donor and recipient blood based on both antigen and antibody presence minimizes the risk of transfusion reactions.
What are the three rules of blood typing?
There aren’t universally agreed-upon “three rules” for ABO blood typing, as it’s a more nuanced process than a set of simple rules. However, here are three key principles that underpin the entire ABO blood typing procedure:
1. Antigen-Antibody Specificity: Each antigen (A, B, or Rh) can only react with its corresponding antibody (anti-A, anti-B, or anti-Rh). This specific binding occurs due to unique shapes on the antigen and antibody molecules that fit together like a lock and key. This forms the basis for identifying antigens and antibodies present in an individual’s blood.
2. Agglutination as Confirmation: When an antigen and its corresponding antibody meet, they bind together, causing the red blood cells (RBCs) to clump together in a visible mass called an agglutinate. This visible reaction confirms the presence of both the antigen and antibody, allowing for interpretation of the blood type.
3. Compatibility for Transfusions: Safe blood transfusions rely on matching the donor’s blood with the recipient’s based on both antigen and antibody compatibility. This means:
- Antigen compatibility: The recipient’s plasma shouldn’t contain antibodies that can react with the donor’s RBC antigens. For example, a recipient with type A blood cannot receive type B blood because their plasma has anti-B antibodies.
- Antibody compatibility: The donor’s plasma shouldn’t contain antibodies that can react with the recipient’s RBC antigens. For example, a type O donor can donate to anyone because their plasma lacks anti-A and anti-B antibodies.
What are the methods of determining the ABO type?
The most common method for determining ABO blood type is the tube agglutination technique. This method has two main components:
- Forward grouping: Red blood cells (RBCs) are tested with known antibodies (anti-A, anti-B, and anti-Rh) to identify antigens present on the RBC surface. Agglutination (clumping of RBCs) indicates a positive reaction and the presence of a specific antigen.
- Reverse grouping: Plasma is tested with known RBCs (type A and B) to detect antibodies present in the plasma. Agglutination again indicates a positive reaction and the presence of a specific antibody.
However, there are a few other methods for determining ABO blood type, although these are less commonly used:
Gel test cards: These pre-made cards contain wells filled with antisera and indicator gel. Adding blood and incubating the card allows for visual interpretation of agglutination reactions within the gel.
Microfluidic devices: These automated systems use microfluidic channels to mix blood samples with antisera and analyze agglutination patterns electronically.
Molecular methods: Techniques like DNA sequencing can analyze the genes responsible for ABO antigens, but this is not yet routinely used in clinical settings due to cost and complexity.
Choosing the appropriate method depends on several factors, including:
- Availability of resources: Tube agglutination is the most widely available and cost-effective method.
- Throughput needs: Microfluidic devices might be preferred for high-volume testing in blood banks.
- Patient factors: In rare cases, specialized techniques like molecular methods might be necessary for individuals with unusual blood types.
What are the different types of antisera used in the tube method, and how do they target specific antigens?
In the tube method of ABO blood typing, several types of antisera are used, each targeting specific antigens on red blood cells.
1. Anti-A and Anti-B Sera
- Source: These are typically produced by immunizing animals (e.g., rabbits, horses) with purified A or B antigens from human red blood cells.
- Anti-A serum contains IgM antibodies that specifically bind to A antigens on red blood cells.
- Anti-B serum contains IgM antibodies that specifically bind to B antigens on red blood cells.
- Mechanism: Antibodies have specific recognition sites that bind to corresponding structures on antigens. When anti-A encounters A antigens, or anti-B encounters B antigens, they bind together, causing agglutination of red blood cells, visible as a clumping reaction.
2. Anti-Rh (D) Serum
- Source: Typically obtained from Rh positive humans who have naturally formed antibodies against the Rh (D) antigen.
- Specificity: Contains IgG antibodies that specifically bind to the D antigen on red blood cells.
- Mechanism: Similar to Anti-A/B, IgG antibodies in Anti-Rh(D) bind to D antigens, causing agglutination if present.
- Subgroup Antisera: These are used to further differentiate within A and B groups (e.g., A1, A2, B subgroups). They are less commonly used in routine ABO typing.
- Anti-Human Globulin (AHG): Used in the indirect antiglobulin test (IAT) to detect weak antigens or antibodies not readily visible in standard agglutination tests.
It’s crucial to understand
- Each antiserum is produced to target a specific antigen and won’t react with other antigens.
- The type of antibody in the antiserum (IgM vs. IgG) affects the reaction kinetics and temperature requirements for optimal agglutination.
- Strict quality control and standardization of antisera are essential for accurate blood typing results.
What are the critical steps and considerations for ensuring accurate results in tube method ABO blood typing?
Ensuring accurate results in ABO blood typing using the tube method requires adhering to critical steps and considerations throughout the process. Here are some key aspects to highlight:
- Patient identification: Double-check patient identifications meticulously at every step to avoid testing the wrong sample.
- Sample collection: Use appropriate tubes (e.g., red-topped for no additive) and follow proper collection techniques to avoid contamination or hemolysis.
- Sample handling: Maintain proper storage and transport temperatures to prevent sample deterioration.
- Standardized protocols: Strictly follow established procedures and quality control measures for all steps, including reagent preparation, pipetting, centrifugation, and incubation times and temperatures.
- Technical proficiency: Ensure technicians are well-trained and experienced in recognizing and interpreting agglutination reactions.
- Reagent quality: Regularly check the validity and performance of antisera and other reagents according to manufacturer’s instructions.
- Microscopic examination: Carefully examine tube reactions under a microscope to differentiate true agglutination from rouleaux or other artifacts.
- Control tests: Include positive and negative controls for each test run to verify the validity of the procedure and reagents.
- Result interpretation: Use clear and standardized criteria for interpreting agglutination patterns, avoiding subjective judgments.
- Double-checking: Two qualified personnel should independently review and document results before finalizing reports.
- Discrepancy management: Investigate and resolve any discrepancies between forward and reverse grouping or inconsistent results with documented history.
- Communication: Clearly communicate results to healthcare providers, highlighting any potential concerns or interpretations.
- Patient factors: Be aware of potential influences like recent immunizations, transfusions, or medications that could affect antigen or antibody expression.
- Rare blood types: Consider using additional tests or consulting specialists for individuals with suspected rare blood types or complex cases.
- Continuing education: Stay updated on the latest guidelines and advancements in ABO blood typing practices.
How does the concentration of antisera and red blood cell suspensions affect the sensitivity of the test?
The concentration of both antisera and red blood cell (RBC) suspensions plays a crucial role in determining the sensitivity of the ABO blood typing test using the tube method.
- Higher concentration: Generally increases the sensitivity of the test. More antibody molecules are available to bind with antigens on RBCs, potentially detecting even weak antigens that might not be visible with lower concentrations. However, excessively high concentrations can lead to non-specific agglutination, causing false positives.
- Lower concentration: Decreases the sensitivity of the test. Fewer antibody molecules are available, potentially missing weak antigens and leading to false negatives. However, a very low concentration is necessary to differentiate weak from strong reactions, crucial for accurate interpretation.
RBC Suspension Concentration
- Higher concentration: Can decrease the sensitivity of the test. With more RBCs, there are fewer antibody molecules per cell, potentially masking weak antigen expression and leading to false negatives. However, a certain minimum concentration is necessary for optimal agglutination to be visible.
- Lower concentration: May increase the sensitivity of the test, especially for detecting weak antigens. However, excessively low concentrations can make it difficult to visualize agglutination, even with positive reactions.
Therefore, finding the optimal balance between antisera and RBC suspension concentrations is crucial for achieving a sensitive and specific ABO blood typing test. This balance is achieved through:
- Standardized protocols: Laboratories use established guidelines and quality control procedures to ensure optimal concentrations are used in each test.
- Reagent adjustments: Manufacturers specify recommended dilution factors for their antisera based on desired sensitivity. Technicians might adjust concentrations slightly based on experience or specific testing conditions.
- Microscopic examination: Careful observation under a microscope allows technicians to distinguish true agglutination from artifacts even with subtle reactions, ensuring accurate interpretation despite concentration variations.
What happens if there is no time for blood typing?
In extremely critical situations when there is absolutely no time for blood typing, medical professionals may resort to administering universal donor blood, also known as O negative (O Rh-) blood. This is because:
- O negative blood lacks both A and B antigens: As a result, individuals with any blood type (A, B, AB, or O) can safely receive O negative blood without triggering an immune response due to incompatibility.
- Rh-negative blood: Additionally, O negative blood is Rh-negative, meaning it lacks the Rh factor antigen. While most individuals (over 85%) are Rh-positive, Rh-negative individuals can also safely receive Rh-negative blood without facing complications.
However, using universal donor blood is not an ideal situation and should only be considered as a last resort due to several potential drawbacks:
- Limited availability: O negative blood is relatively rare, accounting for only about 7% of the population. Relying on it for emergency transfusions can deplete blood bank reserves quickly.
- Increased risk of hemolytic reactions: Although rare, individuals with other blood types might develop delayed hemolytic reactions if they receive multiple transfusions of O negative blood due to the development of antibodies against A and B antigens.
- Potential sensitization: While less likely with a single transfusion, repeated exposure to foreign antigens in O negative blood can sensitize the recipient’s immune system, making it harder to find compatible blood for future transfusions.
Therefore, whenever possible, medical professionals prioritize blood typing and matching the specific blood type of the recipient for transfusions. This ensures maximum compatibility and minimizes the risk of adverse reactions.
Disclaimer: This protocol is intended for informational purposes only and may need to be modified depending on the specific laboratory procedures and patient circumstances. Always consult with a qualified healthcare professional for guidance. See additional information.
- American Association of Blood Banks (AABB). Technical Manual, 21st Edition, 2023.
- Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005.
- Harmening DM. Modern Blood Banking & Transfusion Practices (FA Davis Company). 2018.
- Li HY, Guo K. Blood Group Testing. Front Med (Lausanne). 2022 Feb 11;9:827619. doi: 10.3389/fmed.2022.827619. PMID: 35223922; PMCID: PMC8873177.