phenobarbital in serum/plasma
Indications
- therapeutic drug monitoring, phenobarbital therapy
- epilepsy
- treatment of convulsive seizures
- prevention of febrile seizures
- neonatal hyperbilirubinemia
- lowering bilirubin in chronic cholestasis
Reference interval
- Therapeutic Range: 15-40 ug/mL
- Toxic Range: >40 ug/mL (see phenobarbital for adverse effects)
Principle
Phenobarbital (PB) binds to antiphenobarbital antibody, thereby reducing the amount of phenobarbital G6PD- conjugate (PB-G6PD) that can be bound to the antibody. The PB-G6PD which is not bound to the antibody catalyzes the oxidation of glucose-6-phosphate with the simultaneous reduction of NAD to NADH which absorbs at 340 nm. In the absence of PB, PB-G6PD binds with the antibody; the bound conjugate has less enzymatic activity than the unbound.
Antibody + PB + PB-G6PD ------> Antibody*PB + Antibody* PB-G6PD
+ PB-G6PD (active)
PB-G6PD
Glucose-6-phosphate + NAD --------> 6-phosphogluconolactone
+ NADH
(absorbs at 340 nm)
The increase in absorbance at 340 nm due to the formation of NADH over a 17.07-second measurement period is directly proportional to the activity of the PB-G6PD. The phenobarbital concentration is determined by means of a previously prepared lot specific calibration curve or a mathematical function
Clinical significance
Phenobarbital is used in the treatment of all types of seizures except absence seizure (petit mal). Absorption of oral phenobarbital is slow but complete. The time at which peak plasma concentrations are reached is widely variable & ranges from 4-10 hours after the dose. The elimination half-life is age dependent (children average 70 hours, geriatric patients 100 hours). Since hepatic metabolism is one of the prime routes of elimination, in adults at blood concentrations >40 ug/mL is sedation. Although tolerance to this effect develops with chronic therapy.
Because of the long elimination half-life of phenobarbital, the blood concentration does not change rapidly. Therefore, a serum specimen collected late in the dose interval (trough) is representative of the overall effect. Results from specimens collected 2-4 hours after the dose can be misleading, because they may be construed to be the peak concentration when in actuality they are not.
See phenobarbital for toxicity (adverse effects)
Specimen
Patient Preparation: No special patient preparation is required.
- Serum is the specimen of choice. Specimens are collected in a red top vacutainer & should be centrifuged after collection. Serum samples can be stored at room temperature for several hours. If frozen at -20 C, samples are stable for at least one year. Hemolysis, icterus, & lipemia do not interfere with the PHNO method.
- Plasma specimens are also acceptable. Heparin & EDTA do not interfere with the phenobarbital method. Specimen drawn with heparin should be collected in a green top tube & centrifuged after collection. EDTA samples are collected by venipuncture using a purple top tube & centrifuged after collection.
Minimum sample size: 0.6 milliliters with an optimum size of 1.0 milliliters or larger.
More general terms
More specific terms
Additional terms
References
- ↑ Kaplan, L., & Pesce, A., Clinical Chemistry:theory, analysis, & correlation, C. V. Mosby Co., St. Louis, MO., 1984, pp. 1340.
- ↑ Tietz, N., Fundamentals of Clinical Chemistry, 3rd edition W. B. Saunders Co., Philadelphia, 1987, pp. 852.
- ↑ Tietz, N., Textbook of Clinical Chemistry, W. B. Saunders Co., Philadelphia, 1986, pp. 1632.
- ↑ ACA IV Discrete Clinical Analyzer Instrument Manual, Volume 1:Operation, DuPont Company, Wilmington, Delaware, 1984.
- ↑ ACA IV Discrete Clinical Analyzer Instrument Manual, Volume 3:Chemistry, DuPont Company, Wilmington, Delaware, 1984.
- ↑ Henry's Clinical Diagnosis & Management by Laboratory Methods, 21st edition, McPherson RA & Pincus MR (es), W.B. Saunders Co., Philadelphia, PA. 2007, pg 311
- ↑ Mini Panel of 2 tests: Phenobarbital . Primidone (Mysoline) Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0090202.jsp
- ↑ Phenobarbital Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0090230.jsp