Only MNGIE patients have increased plasma concentrations of both dUrd (>5 [micro]mol/L) and dThd (>3 [micro]mol/L), whereas mutation carriers with partially reduced TP activity have no detectable pyrimidine nucleosides (<0.05 [micro]mol/L) in plasma.
On the basis of our results, we propose an algorithm using measurements of dThd and dUrd in plasma and TP activity in buffy coat to diagnose MNGIE.
This degree of TP deficiency does not alter plasma nucleosides because carriers, like controls, have undetectable amounts of dThd and dUrd.