Calculated PRA (cPRA)

Panel Reactive Antibodies (PRA) has been used to measure patient HLA sensitisation ever since pre-formed donor specific HLA antibodies were associated with hyperacute rejection in renal transplantation in the 1960’s (Patel and Terasaki et.al.). As traditionally defined, PRA refers to the percentage of an antibody screening panel with which the patient’s serum reacts. In the UK, a kidney patient with a PRA > 85% is considered highly sensitised and could potentially benefit from the UK tiered system of kidney allocation which prioritises highly sensitised patients for well matched organs. This measure of PRA however relies on the composition of the panel which may not necessarily reflect the antigen frequencies in the donor population. This measure of PRA is not therefore a good reflection of the chances of the patient finding a compatible donor. Variations in cell panels, both commercial and in house, result in wide variations in recorded PRA for patients on the waiting list.

 

The calculated PRA (cPRA) was introduced to overcome this problem. Developed first in the USA and adopted by UNOS in 2006 and more recently in the UK, the cPRA is based on a calculation of the reaction frequency of a listed set of unacceptable mismatches for a patient, against a panel of 10,000 recently added deceased donors. The cPRA therefore gives a measure of the chances of a patient finding a compatible donor in the donor pool.

 

The use of cPRA based on a set of listed unacceptable mismatches for each patient is made much easier by the wide adoption of solid phase assays such as Luminex. Luminex assays, especially those involving the use of single antigen beads (SABs) allow fine specificity definition and allow the strength of the reactions (MFI) to be used to assess immunological risk and help decide whether or not a specificity should be listed.

 

cPRA removes some of the variability between laboratories using different panels and allows a PRA value to be assigned which reflects the patients transplantability.

 


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