To Obtain Additional Donor Units

1. Based on the population frequency of the corresponding antigen, select an appropriate number of donors of the same ABO and Rh as the patient, and antigen type them for the corresponding antigen.

2. When an appropriate number of antigen negative donors are found, do an antiglobulin crossmatch with the patient . Any antigen-negative donors that are crossmatch-compatible can be released for transfusion.

Formula for calculating number of donors to type

There is a formula that can be used as a guide to calculate the appropriate number of donors that must be antigen typed in order to obtain the required number of antigen-negative donors. For patients with single antibodies, the formula is as follows:

X = number of donors required
frequency of donors negative for the antigen
(expressed as a decimal)

where X = minimum number of donors that must be antigen typed

Examples:

1. Four c-negative donors are required for a patient with anti-c:

   The frequency of the c antigen is 80%.

   x= 4/0.2= 40/2= 20

   Answer: a minimum of 20 donors should be antigen typed.

2. Five e-negative donors are required for a patient with anti-e:

   The frequency of the e antigen is 98%.

   x= 5/0.02= 500/2= 250

   Answer: a minimum of 250 donors should be antigen typed.

   In such a case, the blood center would be asked to find donors who are known to be e-negative.

   For patients with multiple antibodies, the formula is modified as follows:

   X = number of donors required
   (neg frequency of "x" ) X (neg frequency of "y")
   (expressed as a decimals)

   where "x" and "y" are the antigens to which the patient has antibodies.

   Example:

3. Two donors are required for a patient with anti-c and anti-Jk^a:

   The frequency of c is 80% and of Jk^a is 78% .

   x= 2/0.2 X 0.22 = 2/0.044 = 2000/44 = 45.5

   Answer: a minimum of 46 donors should be antigen typed.

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   To obtain additional donors

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