A Relationship between the Granulocyte Phenylalanine Content and the Degree of Disability in Phenylketonuria

A method has been developed to assess clinically the degree of neuropsychiatric damage in phenylketonuria, which takes account of factors other than the intelligence quotient. It can be applied both to severely retarded patients in whom the intelligence quotient is a poor discriminant, and to those whose intelligence can be measured accurately. The numerical value obtained is termed the ‘ability score’. Plasma and intracellular (granulocyte) levels of phenylalanine and the ability scores were measured in 42 cases of phenylketonuria. The ability score correlated significantly with the granulocyte phenylalanine content, but not with the plasma phenylalanine concentration. We suggest that our findings support the concept that high intracellular phenylalanine levels may be a factor in causing brain damage in phenylketonuria. The granulocyte phenylalanine content may be helpful prognostically, and in the control of dietary therapy; and studying the granulocyte as well as the plasma levels may help to clarify the relationship between variant forms of classical phenylketonuria and persistent hyperphenylalaninaemia.