Renal function calculation

Combined determination of serum and urinary values of markers makes it possible to refine the determination of different renal functions. Calculating clearance and excretion values is possible. These calculations are extremely dependent on urine collection, which may be inappropriate. Formulas have been derived to overcome this problem (Tables 28.10, 28.11). 

Calculated fractional excreted sodium (FeNa) less than 1% in patients with compromised baseline renal function, and less than 0.2% in patients with normal baseline renal function indicates dehydration and reduced renal perfusion. 

Methods for calculating volume of distribution (VD) can be influenced by renal disease. Of the commonly used terms (i.e., volumes of central compartment, terminal phase, and distribution at steady state [ Vss]), Vss is the most appropriate for comparing patients with renal insufficiency versus those with normal renal function because Vss is independent of drug elimination. 

Step 4 Calculate individualized treatment regimen Determine treatment goals calculate dosage regimen based on pharmacokinetic characteristics of Ihe drug and the patients renal function... 

Renal function impairment- Upon treatment initiation, the recommended zoledronic acid doses for patients with reduced renal function (mild to moderate renal impairment) are listed in the following table. These doses are calculated to achieve the same AUC as that achieved in patients with Ccr of 75 mL/min. Ccr is calculated using the Cockcroft-Gault formula. 

Drugs cleared by the renal route often require adjustment of clearance in proportion to renal function. This can be conveniently estimated from the creatinine clearance, calculated from a single serum creatinine measurement and the predicted creatinine production rate. 

The decrease of renal function with age is independent of the decrease in creatinine production. Because of the difficulty of obtaining complete urine collections, creatinine clearance calculated in this way is at least as reliable as estimates based on urine collections. Fat-free mass (equation [14]) should be used for obese patients, and correction should be made for muscle wasting in severely ill patients. 

One further point which should be considered is the importance of dose size. Because of the (R) — (S) conversion, the dosage of the (S) form administered may be as much as two or three times the anticipated dose. One can visualize an elderly 90 lb lady, with decreased renal function, who is administered a racemic drug. She receives the normal dose calculated for a 150 lb person (because of the way the tablets are made up). Because of decreased renal function and increased retention there is time for all the (i ) enantiomer to be converted to the (S) enantiomer. Effectively, she will receive three times the needed dose of the active drug and the area under the dose—time curve will be much greater. It is hardly surprising that adverse side effects sometimes occur.99... 

A concern with AUC-targeting based on renal function surrounds the measurement of creatinine clearance. The formulas of Calvert et al. were developed using EDTA clearance, measurement of which is not widely available. They have shown that neither standard measured creatinine clearance, nor the calculation of this index are as accurate or as reproducible. To circumvent this difficulty an alternative dosing strategy has been developed by Chatelut, Canal and co-workers [226], This dosing approach is being tested in clinical trials. 

Calculate Mr VC s renal function using both Cockcroft-Gault and the Modification of Diet in Renal Disease (MDRD) equations. 

Calculate CM s renal function using both the MDRD equation and the... 

The standard equations used to calculate renal function may be applied here, as although the serum creatinine is elevated, it is stable. Were it still changing rapidly, the calculations would not be accurate. 

Depending on the equation used, CM s GFR has been calculated as being between 22 and 26 mL/min. Given this level of renal function, it would be... 

Following the guidelines of the British Hypertension Society, Mr WD should be prescribed a thiazide diuretic and a calcium channel blocker, so a good combination might be nifedipine LA 20 mg or 30 mg once daily or amlodipine 5 mg once daily, plus bendroflumethiazide 2.5 mg once daily. However, we know that his renal function is poor, with a calculated GFR of approximately 25 mL/min, so he is borderline for thiazides to be clinically effective. In this instance, it might be prudent to prescribe an ACE inhibitor instead, for example, enalapril 5-fO mg twice daily. 

We have measured FSH in unextracted urine on an AxSYM random-access immunoassay analyzer (Abbott laboratories, Abbott Park, IL) with a MEIA (microparticle enzyme immuno assay) reagent kit. In order to correct for dilution, creatinine was measured, and the urinary FSH was normalized for creatinine concentration. Urine and serum samples were obtained from 40 women between 32 and 55 years of age. All women were healthy, except for a benign gynecological illness for which they were admitted to our hospital. All women had normal renal function. On the day of operation, we took six serum samples from each patient, each at least an hour apart, in order to calculate the mean serum FSH concentration. During the same day, we collected an early-morning urine sample, 24-h urine sample, and a random void urine sample. 

It is difficult to obtain an accurate measure of renal function in patients with cirrhosis. A number of studies have shown that they tend to have low serum creatinine levels. This has been explained by a reduced muscle mass in cirrhotic patients and a reduced conversion of creatine to creatinine [10]. The calculation of creatinine clearance using the Cockcroft and Gault formula is also inaccurate in predicting GFR in these patients because it uses the serum creatinine level (which may be falsely low) and body weight in the calculation, which is likely to be inflated due to the presence of ascites [12]. The measured creatinine clearance, based on urinary excretion of creatinine, should theoretically be more accurate, even in patients with reduced muscle mass or impaired creatinine synthesis. However, it has been shown that this also overestimates the GFR because of an increased fractional tubular secretion of creatinine in cirrhotic patients, particularly those with reduced GFR [10]. 

FIGURE 2.5 Siimilation of plasma (solid line) and tissue (heavy dashed line) digoxin concentrations after intravenous administration of a 0.75-mg loading dose to a 70-kg patient with normal renal function. Cq is estimated by back extrapolation (dotted line) of elimination-phase plasma concentrations. is calculated by dividing the administered drug dose by this estimate of Cq, as shown. Tissue concentrations are referenced to the apparent distribution volume of a peripheral compartment that represents tissue distribution. (Reproduced with permission from Atkinson AJ Jr, Kushner W. Annu Rev Pharmacol Toxicol 1979 19 105-27.)... 

For digoxhi/ b/2 is usually 1.6 days for patients with normal renal function and k = 0.43 day (0.693/1.6 = 0.43). As a practical point/ it is easier to estimate fi/2 from a graph such as Figure 2.10 and to then calculate k from Equation 2.13/ than to estimate k directly from the slope of the elimination-phase line. 

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