phenytoin in serum/plasma

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[1][2][3][4][5][6]

Indications

Reference interval

  • Therapeutic Range--10-20 ug/mL
  • Toxic Range-- >20 ug/mL

Principle

The unique reagents in this methodology are the matched lots of monoclonal antiphenytoin antibody & the phenytoin-glucose-6- phosphate dehydrogenase conjugate. The reaction sequence, in two steps, is as shown: 

             PTN                  Ab-PTN
1)  Ab +       +      ---------->         +        + Ptn-G6PD
          PTN-G6PD                Ab-PTN-G6PD       (active)
                                 (Inhibited)

2)  Glucose-6-Phosphate               6-Phosphogluconolactone
                        PTN-G6PD
           +           ------------->             +     NAD+             (active)      NADH (absorbs at 340 nm)

    Where:       Ab = Antiphenytoin complex
                PTN = Phenytoin
           PTN-G6PD = Phenytoin - glucose-6-phosphate dehydrogenase conjugate

The concentration of phenytoin determines the amount of phenytoin- glucose-6-phosphate dehydrogenase conjugate (PTN-G6PD) that is bound to the antiphenytoin antibody. The unbound conjugate catalyzes the oxidation of glucose-6-phosphate, with the simultaneous reduction of NAD+ to NADH, more rapidly than does the bound conjugate. The rate of increasing absorbance at 340 nm due to the increase in NADH is related to phenytoin concentration by means of a calibration curve or mathematical function

Clinical significance

Phenytoin is used in the treatment of primary & secondary generalized tonic-clonic seizures, simple-partial seizures or complex-partial seizures, & status epilepticus. It is not effective for absence seizures. Phenytoin is not really soluble in aqueous solutions, & absorption of oral phenytoin is slow & sometimes incomplete. Once absorbed, the drug is highly protein bound (90-95 %). As with all drugs, the pharmacological effect of phenytoin is directly related to the amount present in the free (unbound) state. The degree of protein binding can be reduced by the presence of other drugs, anemia, & the hypoalbuminemia that can occur in the elderly. In these conditions, an increased effect is observed at the same total drug concentration as in plasma from normal patients.

The time to collect the specimen is dictated by the reason for monitoring. If the patient displays any symptoms of intoxication, then the peak blood concentration is of interest. This specimen is collected 4-5 hours after the dose, although the peak level may be delayed up to 8 hours if the drug is given in conjunction with substances that increase stomach acidity. If the principal question is adequate therapy, then the trough concentration is more useful, & the specimen is collected just before the next dose is given.

Specimen

Patient Preparation: No special patient preparation is required.

Minimum sample size 0.6 milliliter: With an optimum size of 1.5 milliliters or larger.

More general terms

More specific terms

Component of

References

  1. ↑ Tietz, N., Fundamentals of Clinical Chemistry, 3rd edition W. B. Saunders Co., Philadelphia, 1987, pp. 852-853.
  2. ↑ Tietz, N., Textbook of Clinical Chemistry, W. B. Saunders Co., Philadelphia, 1986, pp. 1632-1634.
  3. ↑ ACA IV Discrete Clinical Analyzer Instrument Manual, Volume 1:Operation, DuPont Company, Wilmington, Delaware, 1984.
  4. ↑ ACA IV Discrete Clinical Analyzer Instrument Manual, Volume 3:Chemistry, DuPont Company, Wilmington, Delaware, 1984.
  5. ↑ Phenytoin Total Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0090090.jsp
  6. ↑ Panel of 3 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0090141.jsp
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