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1372 Phenobarbital Serum Level Phosphorus

Physiology develop hyposegmentation of neutrophils in Pearls
Segmented neutrophils in dogs and cats typically response to inflammation, B vitamin deficiency, • When an otherwise unremarkable CBC
VetBooks.ir congenital Pelger Huët anomaly exhibit hypose- Pelger Huët). In such cases, hyposegmentation or “metamyelocytes” in an apparently healthy
neoplastic disease, or other illness (“pseudo”
report has a substantial number of “bands”
have 3-5 nuclear lobes. Heterozygotes for the
may be transient.
gmented neutrophils, many of which resemble
dog or cat, consider the likelihood of Pelger
Huët.
band neutrophils. The heterozygous condition
is benign, and affected animals have no ill Reference Interval • Pelger Huët is essentially an incidental
effects with minimal alteration of neutrophil Reported if present on slide review finding, with no adverse clinical effect.
function. The low incidence of homozygotes • Since the hyposegmented neutrophils in cats
is attributed to embryonic death. The genetic Specimen Collection and Handling and dogs with Pelger Huët do not reflect
defect in people is linked to mutations of the EDTA whole blood (lavender top tube) and neutrophilic precursors released from the
laminin B receptor, as B-type laminin mediates freshly prepared blood smear for laboratory marrow, the term “left shift” is not applicable.
nuclear shape by binding the nuclear membrane to stain
to chromatin. Rarely, genetically normal animals AUTHOR: Stephen D. Gaunt, DVM, PhD, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP

Phenobarbital Serum Level

Definition addition/substitution of alternative anticon- textbook or pharmacist for information regard-
Serum levels of anticonvulsant barbiturate used vulsant drug. See p. 903. Consider effects of ing specific drug interactions.
to monitor treated patients comorbidities and concurrently administered
drugs. Evaluate liver enzymes/function (i.e., Lab Artifacts
Physiology bile acids). Avoid serum separator tube, which can cause
Phenobarbital is slowly absorbed from GI falsely decreased phenobarbital concentrations.
system, with good bioavailability (peak 4-8 Causes of Abnormally Low Levels
hours after administration). It undergoes hepatic Serum levels < 10 mcg/mL typically are Specimen Collection and Handling
metabolism with a widely variable elimination subtherapeutic, with considerable variability Serum (red top tube); avoid serum separator
half-life (dogs 12-125 hours, average 48 hours; in responses among dogs. tube. Sample can be collected any time relative
half-life cats 35-56 hours). Diet, body weight to dosing.
and composition, and concurrent drug use Next Diagnostic Steps to Consider
can alter pharmacokinetics. Can result in if Levels are Low Relative Cost: $$
hepatotoxicity Correlate drug levels with seizure control
to decide whether to increase dose, replace/ Pearls
Reference Interval supplement with another anticonvulsant, or • Phenobarbital is involved in many drug
Therapeutic target: 15 to 40 mcg/mL dogs; 23 discontinue. interactions; consult pharmacology textbook
to 28 mcg/mL cats or pharmacist if multiple medications.
Drug Effects • Considerable variability in clinical response
Causes of Abnormally High Levels Concurrent administration of many other drugs, occurs; blood level does not always correlate
> 40 mcg/mL may indicate toxicosis, but there such as certain antibiotics (e.g., doxycycline, with clinical response or hepatic toxicosis.
is considerable variability, and some dogs may chloramphenicol, metronidazole), anticoagu- • Blood levels are not as useful when combin-
develop toxicosis at lower concentrations. lants (warfarin), cardiopulmonary drugs (e.g., ing with bromide or levetiracetam since a
aminophylline/theophylline, certain beta- therapeutic effect likely can be achieved with
Next Diagnostic Steps to Consider blockers, quinidine), glucocorticoids (predni- lower levels of phenobarbital.
if Levels are High sone, others), and many others, alters levels
Consider dose reduction if seizures well- of phenobarbital. Consult a pharmacology AUTHOR: Carrie L. Flint, DVM, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
controlled. If seizures continue, consider

Phosphorus

Definition Physiology Reference Interval
Inorganic form of phosphate; the major Clinical assays measure only the total inor- Dogs: 3.2-8.1 mg/dL. Cats: 3.2-6.5 mg/dL.
intracellular anion; primarily located in bone ganic phosphorus, although both organic and Unit conversion: 1 mg/dL = 0.323 mmol/L
(as hydroxyapatite), with the remainder in soft inorganic phosphorus are present (depending
tissues and in circulation on pH). Blood levels are affected by intestinal Causes of Abnormally High Levels
absorption, shifting between intracellular and Misdistribution (e.g., tumor lysis syndrome,
Synonyms extracellular compartments, renal clearance, myopathies, metabolic alkalosis); increased
2−
Phosphate, PO 4 and animal’s age. Parathyroid hormone (PTH) intake (e.g., phosphate enema, increased
triggers phosphaturia. vitamin D); decreased loss (e.g., decreased renal
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