How Accurate Are Blood Typing Kits?

Blood typing or blood grouping works by detecting a unique set of proteins called antigens present on the surface of red blood cells (RBCs). Your blood type is based on the observation of a simple antigen-antibody response that results in visible hemagglutination, or clumping of RBCs.

The International Society of Blood Transfusion (ISBT) has approved 39 primary human blood type systems to detect common and rare blood group variants. The most common blood grouping systems are ABO and Rh, and together they make eight main blood groups.

Blood is classified into eight major groups based on the presence of various antigens and antibodies:

1. A positive
2. A negative
3. B positive
4. B negative
5. O positive
6. O negative
7. AB positive
8. AB negative

ABO Blood group

The ABO system has four blood groups: A, B, AB, and O. Your blood group is determined by a pair of genes, one inherited from each parent.

• Blood group A: 
  • Antigens A on the red blood cells (RBCs)
  • Antibodies B (Anti-B) that fight against antigen B is present in the plasma
• Blood group B: 
  • Antigens B on the RBCs
  • Antibodies A (Anti-A) that fight against antigen A are present in the plasma
• Blood group O: 
  • Does not contain any antigens on the RBCs
  • Antibodies A and B that fight against both antigen A and B are present in the plasma
• Blood group AB: 
  • Both antigens A and B on the RBCs
  • Antibodies against A and B antigens, which are absent in the plasma

Rh test

Another antigen on the RBCs is called Rh antigen or Rh factor. In the United States, about 85% of people are Rh-positive, and 15% are Rh-negative:

• Rh-positive: Rh antigen is present on the RBCs
• Rh-negative: Rh antigen is absent on the RBCs

ABO testing includes forward and reverse typing.

Forward typing

RBCs contain unique antigens that determine your blood type. When antigen A (Anti-A) or antigen B (Anti-B) reagents are mixed with the RBCs, the antigens on the cells react with the antibodies.

For example, if you are blood type A, your RBCs will clump with antibody A (Anti-A), but not with antibody B (Anti-B). If you are blood type B, your RBCs will clump with antibody B (Anti-B) but not with antibody A (Anti-A). Forward typing findings are used to determine your blood type.

Reverse typing

Serum, or white transparent fluid that is left after separating the RBCs, contains antibodies. When mixed with Anti-A, Anti-B, and Anti-D, the results are opposite to those of forward grouping.

Serum antibodies are mixed with RBCs that contain antigen A (A cells) or antigen B (B cells). The type of antibodies in your serum will decide which reagent red blood cells will cause agglutination, confirming your blood type.

If the sample mixed with reagent A cells is negative, which means agglutination is absent, it implies that you lack the antibody A in your serum; hence, you are blood type A. The reverse typing results are not your blood type, but rather a validation of the forward typing results.

The absence of agglutination indicates that you are blood type O.

Rh test

Everyone is either Rh D positive or Rh D negative.

Anti-D reagent and the red blood cells are used to test for the Rh D factor. If your red blood cells contain the D antigen (Rh D factor), agglutination will occur when tested with antibody D (Anti-D), and you are classified as Rh D positive.

If there is no cell agglutination, you are likely to be Rh D negative.

You inherit your blood type from one of your parents.

Blood typing and compatibility of blood types among couples are required to rule out incompatibility issues between a woman and a child she will conceive. When a woman with Rh-negative blood is pregnant with a baby who has an Rh-positive blood group (because the father is Rh positive), it may cause a condition called Rh incompatibility.

Generally, mother and baby do not mix blood until just after delivery when the cord is cut. However, sometimes a procedure such as amniocentesis or even external cephalic version may cause maternal and fetal blood to come into contact with each other.

If the blood of an Rh-positive infant interacts with an Rh-negative mother, the mother's immune system produces antibodies against Rh and results in a reaction called Rh sensitization. This may cause severe jaundice in the infant. Typically, however, a firstborn child will escape the worst of the disease. If the mother is Rh sensitized, future pregnancies with another Rh-positive baby may cause dire symptoms such as jaundice, liver failure, and even death.

To prevent Rh sensitization, an Rh test is done earlier in pregnancy. If the mother is found to be Rh-negative, she is administered with Rh immunoglobulins.