You should know! #5

Creatinine Clearance

Creatinine comes from the breakdown of creatine phosphate. Creatine phosphate is a breakdown product from muscle. Creatinine is a waste product and is steadily removed by the kidney.

Blood creatinine levels are an indicator of renal function.

Normal serum creatinine levels:

•  0.7 to 1.2 mg/dL (man)
• 0.6 to 1.1 mg/dL (woman)
• Chronic renal insufficiency levels: 1.5-3 mg/dL
• Chronic renal failure: > 3 mg/dL

Creatinine Clearance is used to calculate a patient’s renal functions. Drugs are eliminated by kidney. This calculation is used for adjusting medications doses.

There are many formulas used to calculate creatinine clearance. The Cockcroft- Gault equation is the favoured one used widely in pharmacy calculations.

The kidney removes creatinine. The removal rate is relatively equal to estimated glomerular filtration rate (eGFR). Creatinine clearance estimates renal glomerular filtration rate (GFR). GFR is considered the most accurate index of renal capacity.

Creatinine Clearance is relatively equal to eGFR but may overestimate eGFR by 10-20%;

Creatinine Clearance Cockcroft- Gault Formula (Crcl)

For the Cockcroft-gault formula

1. Use total body weight (TBW) if the patient’s weight is less than ideal body weight (TBW< IBW).
2. Use ideal body weight if total body weight is greater than IBW but < 120% IBW (TBW < 120%).
3. Use Adjusted body weight if total body weight is greater than 120% of IBW or BMI is greater than 25 (TBW > 120%).

Cockcroft-Gault Limitations:

• Only for ≥ 18 years old.
• Overestimates Crcl by 10 to 20 %.
•  Does not correct for ethnicity.
• Does not take into consideration body surface area for obese patients

Chronic Kidney Classification

Be aware of rapid changes in serum creatinine. For example, a rapid increase may be an indication of the patient’s renal function is impaired.  A serum creatinine level increasing greater than 0.5 to 0.7 mg/dL would suggest renal impairment.

Take Note:

Use extreme caution when determining the renal function in the following patients:

• Low body weight
• Liver transplant
• Elderly > 90
• Dehydrated (BUN: serum creatinine ratio > 20:1 indicates dehydration)
• Recent kidney transplant (creatinine is labile.)

Recognize other factors that can impact creatinine clearance:

What can decrease creatinine levels?

• Increased age
• Female patients
• Hispanic patients
• Asians patients
• Amputees
• Small body build
• Decrease muscle mass
• Malnutrition
• Inflammation
• Deconditions (e.g. cancer, severe cardiovascular disease, hospitalized patients)
• Neuromuscular disease (e.g. muscular dystrophy, myasthenia gravis)
• Vegetarian diet
• Advanced liver disease

What can increase creatinine levels?

• Black patients *** (note Africans!!)
• Muscular body build
• Ingestion of cooked meats
• Prerenal (decrease in blood to the kidneys)
• Dehydration
• Shock
• Congestive heart failure
• Hemorrhage
• End stage liver disease
• Renal (Intrinsic factors; damage to renal parenchyma)
• Glomerulonephritis
• Pyelonephritis
• Acute tubular necrosis
• Diabetic nephropathy
• Atheroembolic acute kidney injury
• Rhabdomyolysis
• Acute interstitial nephritis
• Postrenal (factors outside the kidney)
• Ureteral obstruction (genitourinary tumors, stones, fibrosis)

What can decrease creatinine clearance?

• Intravascular depletion (e.g. dehydration, shock, congestive heart failure)
• Acute tubular necrosis
• Intrinsic renal disease (e.g. glomerulonephritis)
• Renal ischemia
• Bladder outlet obstruction
• Drug effect

What can increase creatinine clearance?

• Pregnancy
• Diabetes mellitus (prior to onset of diabetic nephropathy)
• High dietary protein intake
• Strenuous exercise

Chronic kidney disease (CKD) staging system by GFR in mL/min/1.73m²

Empiric Creatinine Clearance

If serum creatinine is unknown, you can adopt the following formula to calculate creatinine clearance.

This formula is based on a few assumptions:

Renal clearance declines by 1% for every year after the age of 30.

Cannot use this formula in patients with non- age related causes of renal dysfunction (e.g. acute kidney disease (AKI), chronic renal disease (CKD), Acute tubular necrosis (ATN), etc.)

Formula:

Male: CrCL= 125 X [0.99^(Age-30)]

Female= CrCL= 115 X [0.99^(Age-30)]