Renal Laboratory Testing

Serum Creatinine: Creatine metabolism in skeletal muscle produces creatinine; creatinine is also derived from dietary meat. Creatinine is normally filtered out of the blood by the kidneys. Serum creatinine levels vary inversely with GFR.

• Normal values males: 0.7-1.3 mg/dL
• Normal values females: 0.6-1.1 mg/dL

Increased levels: kidney damage/failure. Be aware serum creatinine can also be lower in those with low muscle mass (i.e., the elderly). Creatinine Clearance: 24-hour test to evaluate proteinuria, albuminuria, microalbuminuria. Creatinine clearance is doubled for every 50% reduction in GFR. Calculate creatinine clearance here.. Estimated Glomerular Filtration Rate: Gold standard for determining kidney function. eGFR is used to determine stage of chronic kidney disease

• Normal values: >90 mL/min
• CKD: Three months or more at less than 60 mL/min.

Calculate eGFR with serum creatinine, age, and sex.

• Calculate eGFR

Blood Urea Nitrogen: BUN is a measure of how well the kidneys excrete urea, which is a waste product of protein metabolism). Urea levels rise when renal blood flow is reduced or if the kidneys are damaged, but BUN is not as sensitive as serum creatinine or eGFR. BUN-to-Creatinine Ratio: This ratio lowers when renal blood flow lowers. The ratio is used to evaluate dehydration, hypovolemia, and acute kidney failure, and can be useful for determining the cause of failure (renal, intrarenal, postrenal). Urinalysis: Gross evaluation, dipstick analysis, microscopic exam of urine sediments.

• Large amounts of epithelial cells: contamination

• White blood cells (leukocytes): Normal is up to 5 WBC/hpf. Leukocyte esterase is a marker for the presence of WBC. Neutrophils are commonly associated with bacteria. Leukocytes in urine is called pyuria; common in acute cystitis in males.

• Red blood cells: A few are normal. Gross hematuria when blood is visible in the urine (pink to brown appearance). Beware of contamination from menses or other vaginal discharge, semen, hemorrhoids, or rectal bleeding.

• Protein: Persistent protein in the blood suggests chronic kidney damage. Transient proteinuria is common; benign causes include fever, intense physical activity, acute illness, and dehydration. Protein may also be present in acute pyelonephritis.

• Nitrates: Indicative of UTI.

• Casts: Formed in the nephron tubules, so shaped like cylinders. Hyaline casts are nonspecific; WBC casts can be seen in infection or inflammation; RBC casts are due to microscopic bleeding (check for glomerulonephritis).

• pH: Normal is 4.5 to 8. Useful to evaluate kidney stones and infections.

Protein diet is associated with higher acidity; citrus and low-carbohydrate diets are associated with lower acidity.