To the Editor.—The cross-reactivity of phenytoin metabolites that can accumulate in conditions such as renal insufficiency is an important issue that can potentially lead to inaccurate phenytoin measurements and consequent mismanagement of phenytoin therapy. The recent article by Soldin et al,1 “Phenytoin Overview—Metabolite Interference in Some Immunoassays Could be Clinically Important,” seems to focus on the wrong metabolite. It describes an investigation of immunoassay cross-reactivity of the phenytoin metabolite, 5-p-hydroxyphenyl-5-phenylhydantoin (HPPH), when added to a specimen at a concentration of 5 μg/mL. The authors state, “It would be expected that the concentration of HPPH, which is excreted renally, would be significantly elevated in these individuals.” While HPPH is the primary metabolite of phenytoin, its clearance is primarily by formation of its O-glucuronide, rather than by renal elimination. In one study, HPPH concentrations in subjects with chronic renal failure were approximately twice those seen in normal control subjects, but were consistently less than 10% of the phenytoin concentration.2 In another, HPPH concentrations never exceeded 1.5 μg/mL in patients with end-stage renal disease.3 Supplementing specimens with 5 μg/mL of HPPH seems likely to substantially overestimate the HPPH concentrations that will be observed clinically. In contrast, elimination of HPPH-glucuronide is almost entirely renal. Concentrations of HPPH-glucuronide were increased 10-fold in renal failure, reaching levels 4 to 10 times higher than the phenytoin concentration reported in one study2 and had a median predialysis value of 32 μg/mL in the second.3
A more appropriate study design might have been to supplement 1 or more proficiency testing samples with HPPH-glucuronide. Unfortunately, HPPH-glucuronide is not commercially available, making this type of study difficult to implement. The cross-reactivity of HPPH and HPPH-glucuronide have been assessed for the TDx phenytoin assay.4 HPPH at a concentration of 10 mg/L had a cross-reactivity of 15.9%, in agreement with the study by Soldin et al,1 while HPPH-glucuronide had an intrinsic cross-reactivity of 1.6% and an additional cross-reactivity of 0.18% for each 1 mg/L phenytoin present.
A number of minor phenytoin metabolites have been found in urine, including 5-(3′-hydroxyphenyl)-5-phenylhydantoin, 5-(3′,4′-dihydroxyphenyl)-5-phenylhydantoin, and phenytoin-N-glucuronide.5,6 Little is known about serum levels of these metabolites or to what extent they may accumulate in renal failure.
A study in which phenytoin metabolites are quantified in patients with chronic renal failure receiving phenytoin is needed. Modern analytical methods can enable these measurements to be made easily and accurately. The relative concentrations of HPPH, HPPH-glucuronide, and other metabolites could be measured and correlated with renal status, including the use of hemodialysis and peritoneal dialysis. Once this information was thoroughly documented, the appropriate metabolites could be synthesized. Then proficiency testing survey material could be supplemented with both phenytoin and the appropriate concentrations of phenytoin metabolites that do accumulate in patients with renal failure.