Her higher glomerular filtration rate (GFR), expanded extracellular fluid volume, and anabolism in the developing fetus contribute to her relatively low BUN of 5 to 7 mg/dl.
The normal serum creatinine (sCr) varies with the subject's body muscle mass and with the technique used to measure it.
For the adult male, the normal range is 0.6 to 1.2 mg/dl, or 53 to 106 μmol/L by the kinetic or enzymatic method, and 0.8 to 1.5 mg/dl, or 70 to 133 μmol/L by the older manual Jaffé reaction.
It still has limited specificity, however, as illustrated by spurious elevations with sulfonylurea compounds, and by colorimetric interference from hemoglobin when whole blood is used.
This reaction is nonspecific and subject to interference from many noncreatinine chromogens, including acetone, acetoacetate, pyruvate, ascorbic acid, glucose, cephalosporins, barbiturates, and protein.
For example, the recent kinetic-rate modification, which isolates the brief time interval during which only true creatinine contributes to total color formation, is the basis of the Astra modular system.
Usually, a small amount (less than 0.5 g/day) is lost through the gastrointestinal tract, lungs, and skin; during exercise, a substantial fraction may be excreted in sweat.
Reabsorption is also increased by volume contraction, reduced renal plasma flow as in congestive heart failure, and decreased glomerular filtration.
Creatinine formation begins with the transamidination from arginine to glycine to form glycocyamine or guanidoacetic acid (GAA).
The ratio is useful for the diagnosis of bleeding from the gastrointestinal (GI) tract in patients who do not present with overt vomiting of blood.
[8] A common assumption is that the ratio is elevated because of amino acid digestion, since blood (excluding water) consists largely of the protein hemoglobin and is broken down by digestive enzymes of the upper GI tract into amino acids, which are then reabsorbed in the GI tract and broken down into urea.
However, elevated BUN:Cr ratios are not observed when other high protein loads (e.g., steak) are consumed.
[citation needed] Renal hypoperfusion secondary to the blood lost from the GI bleed has been postulated to explain the elevated BUN:Cr ratio.
[10] Hypercatabolic states, high-dose glucocorticoids, and resorption of large hematomas have all been cited as causes of a disproportionate rise in BUN relative to the creatinine.