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Blood Urea Nitrogen (BUN) Bone Marrow Cytology 1317

Blood Urea Nitrogen (BUN)
VetBooks.ir Definition

the GI tract, high protein diet
Concentration of urea in serum, plasma, or • Increased protein digestion: hemorrhage into panel for evidence of hepatocellular injury
and/or decreased liver function. Assess urine
whole blood (usually measured in serum) specific gravity, urine output, water intake,
Next Diagnostic Steps to Consider serum concentrations of electrolytes.
Synonyms if Levels Are High
BUN, urea, serum urea nitrogen (SUN) Evaluate with creatinine as these values often Specimen Collection and Handling
parallel. If creatinine is not increased, consider Serum (red top tube) preferred. Heparinized
Physiology early azotemia, increased protein catabolism, plasma (green top tube) acceptable. Some
Most of serum urea is synthesized via the hepatic gastrointestinal hemorrhage, or dietary cause. methods can use EDTA plasma (lavender top
urea cycle, which converts ammonium derived Also consider with urine specific gravity. Well- tube).
from bacterial metabolism of dietary protein concentrated urine suggests prerenal causes.
to urea. Minimal ammonium is derived from Isosthenuric or minimally concentrated urine Relative Cost: $$ (part of chemistry or renal
deamination of amino acids. The kidneys excrete supports renal and postrenal etiologies, as well as panel); $ (single test)
urea. Urea is freely filtered by glomeruli; some conditions that may impair urine-concentrating
is resorbed by tubular epithelial cells to establish ability (e.g., hypoadrenocorticism, diuretic use, Pearls
(with sodium) the medullary concentration hypercalcemia, diabetes insipidus). • High doses of gastroduodenal ulcerogenic
gradient. drugs (corticosteroids, nonsteroidal anti-
Causes of Abnormally Low Levels inflammatory drugs) may cause intestinal
Reference Interval Hepatic dysfunction (e.g., liver disease, por- hemorrhage, increasing BUN with little effect
• Dogs: 10-30 mg/dL (3.6-11 mmol/L) tosystemic shunt), increased urea excretion on creatinine.
• Cats: 15-35 mg/dL (5.4-12.5 mmol/L) (polyuria of any cause), decreased dietary • Increased BUN with concurrent isosthenuria
protein (including low-protein therapeutic raises concern for compromised renal func-
Causes of Abnormally High Levels diets), physiologic (e.g., young animals), tion, but other causes of impaired urine
• Decreased glomerular filtration rate (GFR): congenital deficiency of urea cycle enzymes concentration ability/ADH responsiveness
azotemia from prerenal causes (e.g., dehydra- (rare) (e.g., hypoadrenocorticism, hypercalcemia,
tion, decreased cardiac output, shock), renal diuretic use, diabetes insipidus) must also
disease, or postrenal causes (e.g., urinary Next Diagnotic Steps to Consider if be considered.
outflow obstruction, rupture) Levels Are Low
• Increased protein catabolism: fever, starva- Check history for polyuria/polydipsia and AUTHOR: Mary Leissinger, DVM, MS, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
tion, exercise information on diet. Check the chemistry

Laboratory Tests

Bone Marrow Cytology

Definition myeloid/erythroid ratio (M:E) is ≈1:1, and can CBC, serum biochemistry profile, and other
Microscopic evaluation allowing assessment of range from 0.6:1 to 4:1. diagnostic tests
blood precursor cell production and morphol-
ogy. See p. 1068 for bone marrow aspiration Causes of Abnormally High Levels Causes of Abnormally Low Levels
protocol. Hyperplasia of one or more cell lines may • Marrow hypoplasia indicates decreased
occur secondary to increased peripheral cell production. Causes include anemia
Physiology demand for cells due to cell destruction, of chronic disease, chronic kidney disease,
Bone marrow contains trabecular bone with increased utilization, or loss. Neoplasia such ehrlichiosis (dogs), drug-induced or immune-
marrow spaces containing capillaries, hema- as acute or chronic leukemia (with or without mediated destruction of precursor cells in the
topoietic cells, and adipocytes. Hematopoi- maturation and morphologic abnormali- marrow, toxic insult (e.g., sulfa antibiotics,
etic activity occurs in response to peripheral ties) results in high marrow cellularity and albendazole, phenylbutazone), viral infection
demand. Bone marrow aspiration is indicated effacement of normal marrow tissue. Lym- (i.e., feline leukemia virus, parvovirus, canine
when there are unexplained cytopenias phocytes and plasma cells may be increased distemper), myelofibrosis.
(nonregenerative anemia, neutropenia, and/ in marrow because of antigenic stimulation • Maturation arrest and dysplastic changes
or thrombocytopenia), hyperglobulinemia, or neoplasia. Iron stores may be increased of erythrocytes occur with viral infection,
suspicion of certain types of neoplasia, or as with anemia of chronic disease and hemolytic iron deficiency, myelodysplastic syndrome,
part of staging of confirmed lymphoma and anemia. Inflammation, infiltrating neoplasia, or toxic insult.
systemic mastocytosis. or hemophagocytic disease also results in • Iron stores are decreased with blood loss and
increased cellularity of the marrow. iron deficiency.
Reference Interval
Marrow particles should contain 25%-75% Next Diagnostic Steps to Consider Next Diagnostic Steps to Consider
hematopoietic cells, with all cell lines exhibiting if Levels Are High if Levels Are Low
orderly, complete maturation and normal shape, Dependent on differential diagnosis based on Dependent on differential diagnosis based on
size, and features (morphology). The normal history, physical exam findings, and results of history, physical exam findings, and results of

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