Title 13. RENAL & THYROID FUNCTION TESTS.pdf ✅

PHARMD GURU Page 1 RENAL FUNCTION TESTS INTRODUCTION:  Renal function may be assessed by measuring blood urea nitrogen (BUN) and serum creatinine. Renal function decreases with age, which must be taken into account when interpreting test values. a) These tests primarily evaluate glomerular function by assessing the glomerular filtration rate (GFR). b) In many renal diseases, urea and creatinine accumulate in the blood because they are not excreted properly. c) These tests also aid in determining drug dosage for drugs excreted through the kidneys.  Azotaemia describes excessive retention of nitrogenous waste products (BUN and creatinine) in the blood. The clinical syndrome resulting from decreased renal function and azotaemia is called uraemia. a) Renal azotaemia results from renal disease, such as glomerulonephritis and chronic pyelonephritis. b) Pre-renal azotemia results from such conditions as severe dehydration, hemorrhagic shock, and excessive protein intake. c) Post-renal azotemia results from such conditions as ureteral or urethral stones or tumours and prostatic obstructions.  Clearance—a theoretical concept defined as the volume of plasma from which a measured amount of substance can be completely eliminated, or cleared, into the urine per unit time - can be used to estimate glomerular function. CLINICAL LABORATORY TESTS USED IN THE EVALUATION OF DISEASE STATES, AND INTERPRETATION OF TEST RESULTS

PHARMD GURU Page 3 CREATININE CLEARANCE:  Creatinine clearance, which represents the rate at which creatinine is removed from the blood by the kidneys, roughly approximates the GFR. a) The value is given in units of millilitres per minute, representing the volume of blood cleared of creatinine by the kidney per minute. b) Normal values for men range from 75 to 125 mL/min.  Calculation requires knowledge of urinary creatinine excretion (usually over 24hrs) and concurrent serum creatinine levels. Creatinine clearance is calculated as follows: Where ClCR is the creatinine clearance in millilitres per minute, CU is the concentration of creatinine in the urine, V is the volume of urine (in millilitres per minute of urine formed over the collection period), and CCR is the serum creatinine concentration.  Suppose the serum creatinine concentration is 1 mg/dL, and 1440 mL of urine was collected in 24hrs (1440mins) for a urine volume of 1 mL/min. The urine contains 100 mg/dL of creatinine. Creatinine clearance is calculated as:  Incomplete bladder emptying and other problems may interfere with obtaining an accurate timed urine specimen. Thus, estimations of creatinine clearance may be necessary. These estimations require only a serum creatinine value. One estimation uses the method of Cockcroft and Gault, which is based on body weight, age, and gender.
PHARMD GURU Page 4 a. This formula provides an estimated value, calculated for males as: b. For females, use 0.85 of the value calculated for males. Example: A 20-year-old man weighing 72 kg has a CCR of 1.0 mg/dL; thus  Determination of GFR. The modified diet in renal disease (MDRD) equation is considered a more accurate measurement of GFR than other equations used to estimate renal function (e.g., Cockcroft –Gault) in patients with reduced GFR and is used in staging renal disease. Patients must have a serum creatinine concentration. a) The MDRD equation for males is as follows: Where, Pcr is serum creatinine. For females, multiply the result by 0.742; for African Americans,multiply by 1.210. b) The MDRD has been validated in Caucasians, patients with diabetic kidney disease, kidney transplant recipients, and African Americans and Asians with non- diabetic kidney disease. c) Many institutions are routinely reporting MDRD-derived GFR estimation for patients as a routine component of a blood chemistry study. This value should be used to assist the clinician in staging a patient’s degree of renal dysfunction and is not a substitute for creatinine clearance as estimated by the Cockcroft and Gault equation, which should be used for drug dosing in renal impairment. The MDRD estimate has not been evaluated for the purpose of drug dosing.