Fractional excretion, normal values

The most useful way to utilize urine electrolyte information is to calculate the fractional excretion (FE), which is the proportion of the filtered load that is excreted from the plasma. If both tubular function and plasma electrolyte values are normal, increases in electrolyte FE values clearly reflect a decrement in GFR. With tubular malfunction, the direction of the change in FE values depends on the net direction of electrolyte transport (i.e., FE will increase for electrolytes that are primarily reabsorbed and will decrease for secreted electrolytes) (Finco 1997 Stockham and Scott 2002). 

Abnormal values of fractional lithium excretion have been reported in a variety of conditions. In hyperthyroidism and Bartter s syndrome fractional hthium clearance is increased. After unilateral nephrectomy, hthium clearance by the remaining kidney increases. After two weeks, fractional lithium clearance returns to normal. Rombola et al. [8] reported markedly increased fractional hthium clearance values in patients with Fanconi syndrome, renal glycosuria, and hypercalciuria. 

In Equation 1.36, Dg is the adjusted maintenance dose for the patient with renal failure, D is the maintenance dose for a normal individual,/is the fraction excreted, and C/r is the patient s creatinine clearance, which can be determined from the serum creatinine value using the Cockcroft and Gault Equation 1.42. If a new dosing interval for the patient with renal failure (x ) is calculated, a reciprocal of the bracketed term in Equation 1.45 is used and the new equation becomes Equation 1.46. ... 

Measurement of fecal excretion of isotopic bile acids (65) gives only the half-life of the labeled bile acid used. The isotope is injected intravenously, and the daily fecal excretion of radioactivity is measured. According to this procedure, the fractional excretion rate of cholic acid in man is normally about 12-13% per day (66,67). Disadvantages of the method are that the absolute values are not obtained, the cholic and chenodeoxycholic acid excretions must be measured separately or a double label method must be used, and the fecal flow should be regular, though an unabsorbable fecal marker can be used. The method appears to be suitable for screening of ileal dysfunction. 

The patient was placed on an essentially purine-free diet and received no medication for 5 days. Uric acid metabolism disclosed uricemia 2.0 mg/dl uricosuria 510 mg/day Cur 15.4 ml/min Ccr 91 ml/min fractional excretion of uric acid (Cur/Ccr) 16.9%. Basal plasma calcitonin was 168 pg/ml (normal values undetectable). A pentagastrin bolus injection of 0.5 g/Kg elevated plasma calcitonin over 1000 pg/ml. Simultaneously, serum uric acid decreased from 2.0 mg/dl to 1.3 mg/dl, and Cur/Ccr increased from 16.9% to 25.7%. A pentagastrin test in two control subjects did not make plasma calcitonin levels detectable, nor did it modify uric acid excretion. Her clinical course was progressively down-hill and she died after several bronchoneumonic episodes and massive tracheal hemorrage. Pyrazinamide and probenecid tests could not be done. Permission for autopsy was denied. 

Goodman and Gilman (11) or other reference sources to obtain values of CLe and the fractional dose eliminated by renal excretion (percentage urinary excretion) in normal subjects. 

In contrast to flecainide, the renal clearance values [117,127] of the enantiomers of tocainide are less than the expected value ( lOOmL/min) based on a normal glomerular filtration rate and the drug free fraction (0.85-0.9, Table 2), suggesting involvement of tubular reabsorption for the enantiomers. In agreement with the lack of substantial stereoselectivity in plasma protein binding (Table 2) and the nonstereoselective mechanisms involved in renal excretion of the enantiomers, the renal clearance of S(-l-)-tocainide (50 mL/min/70 kg) is very close to that of R(—)-tocainide (57 mL/min/70 kg) [127]. 

As shown in Table 2, the compounds formed by transaminative catabolism of cysteine represent only a minor fraction (about 0.5 ) of all sulfur compounds present in normal human urine. It should be noted that the values given for thiocyanate were obtained on non-smoking subjects, as tobacco-smoking results in an elevated thiocyanate excretion. 

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