Page 206 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

196 ELECTROLYTE DISORDERS

BODY STORES AND Hemolysis may affect laboratory results because
DISTRIBUTION phosphate is present in erythrocytes. Human erythrocytes
contain 8 mmol/dL red cells, whereas canine erythrocy-
Phosphate is the body’s major intracellular anion, and tes contain 35 mmol/dL and feline erythrocytes contain 26
30
translocation in and out of the intracellular compartment mmol/dL. Hyperlipidemia and hyperproteinemia some-
can rapidly change serum phosphorus concentration. times cause overestimation of serum phosphorus concen-
27,68,97
Gradual changes in total body phosphate can be tration, depending on the methodology used. This
accommodated without noticeable changes in serum can become important when using drugs such as liposomal
95
phosphorus concentration, resembling the situation with amphotericin B. Thrombocytosis and monoclonal
potassium (the major intracellular cation). Approximately gammopathy also may cause spurious increases in serum
92,103,108
80% to 85% of total body phosphate is inorganic hydroxy- phosphorus concentration. Mannitol and other
apatite in bone, whereas 15% is in soft tissues such as mus- drugs may interfere with some assay systems, leading to
cle. 59,88 Most soft tissue phosphorus is organic and can be erroneous measured values. 62,179 Icterus and hemolysis
readily converted to the inorganic form as needed. The were reported to result in artifactual hypophosphatemia
ECF compartment contains less than 1% of total body in dogs with immune-mediated hemolytic anemia. 72
phosphorus stores. Artifactual hypophosphatemia can occur in some
automated systems but not in others. Thus, occurrence
of hypophosphatemia in patients without known
NORMAL SERUM predisposing factors should prompt consideration of
CONCENTRATIONS laboratory error.

Normal serum phosphorus concentrations in adult dogs DIETARY INTAKE
range from 2.5 to 6.0 mg/dL, but they are higher in dogs
younger than 1 year. 17,77,131,175 Serum phosphorus The average phosphorus content of commercial pet foods
concentrations are highest in puppies less than 8 weeks is approximately 1% on a dry matter basis. Dogs and cats
of age (up to 10.8 mg/dL may be considered normal) need to ingest 0.5 to 3.0 g of phosphorus per day,
and gradually decrease into the adult range after 1 year depending on their body size and energy requirements.
of age. 73 Sex-related changes are not reported. 134 The The source of dietary phosphorus markedly affects
effect of age is less pronounced in cats, but immature cats absorption and excretion of phosphorus in cats. 56 The
have a tendency for higher serum concentrations. 32 Bone amount of phosphorus absorbed by the gastrointestinal
growth and an increase in renal tubular reabsorption of tract, the amount excreted in the urine, and the extent
phosphorus mediated by growth hormone presumably of postprandial hyperphosphatemia were increased when
contribute to this age effect. Feeding also affects serum monobasic and dibasic salts of phosphorus were fed but
phosphorus concentration. A carbohydrate meal or infu- decreased when phosphorus originated from poultry,
sion (e.g., 5% dextrose) decreases serum phosphorus con- meat, and fish meal.
centration, because phosphate shifts into intracellular
fluid as a result of stimulation of glycolysis and formation INTESTINAL ABSORPTION
of phosphorylated glycolytic intermediates in muscle,
liver,andadiposecells.Incontrast,proteinintakeincreases Ingested organic phosphate is hydrolyzed in the gastroin-
serum phosphorus concentration because of the relatively testinal tract, liberating inorganic phosphate for absorp-
high phosphorus content of protein-rich diets. tion. Net intestinal phosphate absorption (i.e., the
Time of sampling affects the observed serum phospho- difference between dietary and fecal phosphate) is
rus concentration. People have substantial variations in approximately 60% to 70% of the ingested load, and
serum phosphorus concentrations throughout the absorption is a linear function of phosphorus intake. In
day. 99 Acid-base balance also influences serum phospho- an animal in zero phosphorus balance, urinary phosphate
rus concentration. Respiratory alkalosis stimulates glycol- excretion equals net intestinal phosphate absorption.
ysis (by activating phosphofructokinase) and decreases Intestinal phosphate absorption occurs via two
serum phosphorus concentration. Thus, the measured mechanisms. Passive diffusion is the principal route and
serum phosphorus concentration is affected by several occurs primarily through the paracellular pathway. Active
variables and does not accurately indicate total body mucosal phosphate transport is a sodium-dependent, sat-
phosphorus stores. Measuring serum phosphorus con- urable carrier-mediated process. Calcitriol (1,25-
centration after a 12-hour fast minimizes confounding dihydroxycholecalciferol) increases active intestinal
factors, but the clinician must understand that the mag- mucosal phosphate transport, but this mechanism is
nitude of hypophosphatemia or hyperphosphatemia probably important only during dietary phosphate defi-
may be incorrectly assessed if only one serum or plasma ciency. Both transport mechanisms function in the duo-
sample is analyzed. denum, whereas diffusion is the primary mechanism in