Renal System Review
Patient presents with acute onset of high fever, chills, dysuria, frequency, and unilateral flank pain. The flank pain is described as a deep ache. May complain of nausea (with/without vomiting). May have recent history of urinary tract infection (UTI).
Acute Renal Failure
Patient presents with the abrupt onset of oliguria, edema, and weight gain (fluid retention). Complains of lethargy, nausea, and loss of appetite.
Rapid decrease in renal function. Elevated urinary and serum creatinine. During the early stages of severe acute renal failure, the serum creatinine and the estimated GFR may not accurately reflect true renal function.
Elderly male patient (median age at diagnosis: 73 years), a smoker, presents with painless hematuria. The hematuria can be microscopic or gross (urine pink to reddish color). Some patients only notice a problem after they see a blood-tinged stain on underwear (males, menopausal females). The hematuria may only appear at the end of voiding. May have irritative voiding symptoms (dysuria, frequency, nocturia) that are not related to a UTI. Order a UA, urine C&S, and urine for cytology. Patients who have advanced disease with metastases may complain of lower abdominal or pelvic pain, perineal pain, low-back pain, or bone pain.
The kidneys are located in the retroperitoneal area. The right kidney is lower than the left kidney because of displacement by the liver. The basic functional units of the kidney are the nephrons, which contain the glomeruli.
Kidneys are the body’s regulators of electrolytes and fluids. Water is reabsorbed back into the body by the action of antidiuretic hormone and aldosterone. Kidneys excrete water-soluble waste products of metabolism (i.e., creatinine, urea, uric acid) into the urine. They also produce the hormone erythropoietin, which stimulates bone marrow into producing more RBCs. 252The average daily urine output is 1,500 mL. Oliguria is defined as a urinary output of less than 400 mL per day (adults). Kidneys also secrete several hormones such as erythropoietin (RBC production), renin and bradykinin (blood pressure), prostaglandins (renal perfusion), and calcitriol/vitamin D3 (bone).
Male: 0.7 to 1.3 mg/dL
Female: 0.6 to 1.1 mg/dL
When renal function decreases, the creatinine level will increase. Creatinine is the end-product of creatine metabolism, which comes mostly from muscle. Serum creatinine may be falsely decreased in people with low muscle volume (elderly). Elevated values are seen with renal damage or failure, nephrotoxic drugs, etc. Factors that affect the serum creatinine are gender (males have higher levels), race (African Americans have more muscle mass), and muscle mass.
Creatinine Clearance (24-Hour Urine)
When renal function decreases, the creatinine clearance also decreases. This test is ordered to evaluate patients with proteinuria, albuminuria, and microalbuminuria. It is a more sensitive test than the serum creatinine alone because it reflects the renal function within a 24-hour period. Creatinine clearance is relatively constant and is not affected by fluid status, diet, or exercise. Creatinine clearance is doubled for every 50% reduction of the GFR.
Estimated Glomerular Filtration Rate (eGFR)
Normal: eGFR greater than 90 mL/min
Renal failure: eGFR less than 15 mL/min (stage 5 chronic kidney disease)
The eGFR is the number derived by using the serum creatinine in a prediction equation (i.e., Cockcroft–Gault). The more damaged the kidneys, the lower the eGFR value. The GFR is the amount of fluid filtered by the glomerulus within a certain unit of time. It is used to evaluate renal function and to stage chronic kidney disease.
Best if patient does not eat meat 12 hours before the blood test
GFR is less reliable (interpret with care): Drastic increase/reduction muscle mass (bodybuilders, amputees, wasting disorders), pregnancy, and acute renal failure
Blood Urea Nitrogen (BUN)
Among patients with heart failure, lower GFR with higher BUN is associated with higher mortality. The BUN is not as sensitive as the serum creatinine or the GFR. A high BUN may be caused by acute renal failure, high-protein diet, hemolysis, congestive heart failure, or drugs. If a patient has an abnormal BUN level, check the eGFR. If the eGFR is normal, the renal function is probably normal. The BUN is a measure of the kidney’s ability to excrete urea (waste product of protein metabolism).
The ratio between the BUN and serum creatinine (BUN:Cr). It is used to help evaluate dehydration, hypovolemia, acute renal failure, and it is useful for classifying the type of renal failure (renal, infrarenal, or postrenal).
Large amounts in a urine sample indicate contamination
A few epithelial cells are considered normal
Normal WBCs in urine: less than or equal to 10 WBCs/mL
Called leukocyte esterase test with dipstick strips
Presence of leukocytes in urine (pyuria) is always abnormal in males (infection)
UA is a more sensitive test for infection in males than females
Urine for Culture and Sensitivity
Greater than or equal to 105 colony-forming units (CFU)/mL of bacteria of one dominant bacteria (usually Escherichia coli) are indicative of a UTI
If multiple bacteria are present, it is considered a contaminated sample
Lower values are indicative of bacteriuria
Red Blood Cells
Few RBCs (<5 cells) is considered normal
Hematuria is seen with kidney stones, pyelonephritis, and sometimes in cystitis
Can be contaminated by menses or hemorrhoids
Indicates kidney damage (chronic kidney disease)
May be present in acute pyelonephritis (resolves after treatment)
Urine dipsticks detect only albumin, not microalbumin (Bence-Jones proteins)
Order 24-hour urine for protein and creatinine clearance
Indicative of infection with E. coli
Due to breakdown of nitrates to nitrites by certain bacteria
Casts are shaped like cylinders because they are formed in the renal tubules
Hyaline casts are “normal” and may be seen in concentrated urine
WBC cast may be seen with infections (UTI, pyelonephritis)
RBC casts and proteinuria are diagnostic of glomerulonephritis
Microscopic hematuria is revealed by a positive urine dipstick for heme or by microscopic UA (presence of three to five RBCs or more per high-power field). It can be transient or persistent. Suspect gross (or visible) hematuria if color of urine is pink, red, or brown or blood clots are present. If dipstick is heme positive, next step is to order a microscopic UA. If infection is suspected, order urine for C&S. If malignancy is suspected, send urine for cytology. Risk factors for urothelial or renal malignancy are age older than 50 years, male, smoker, and gross hematuria.
No antibiotic treatment is recommended for patients with Foley catheters, suprapubic, or condom catheters (chronic or intermittent), the elderly (institutionalized), and those with spinal cord injury. Defined as urine culture growth of 100,000 CFU/mL or more. Symptomatic bacteriuria (leukocytosis, fever, chills, malaise) is treated as a UTI. Always treat pregnant women with antibiotics (up to 30% risk of pyelonephritis).
Urinary Tract Infections
Cystitis (urinary bladder inflammation) can be uncomplicated, recurrent, a reinfection, or relapse. The majority of infections are caused by E. coli (75%–95%). Other causal agents are Staphylococcus saprophyticus, Proteus mirabilis, and Klebsiella pneumoniae. UTIs in children younger than age 3 and pregnant women (20%–40% chance) are more likely to progress to pyelonephritis.
Infancy: UTIs are common in boys (usually due to anatomical abnormality)
Children: UTIs in children need further evaluation
Females: Highest incidence is during the reproductive-age years
Female gender; pregnancy
History of a recent UTI or history of recurrent infections
Diabetes mellitus (or immunocompromised status)
Failure to void after sex or increased sexual intercourse (i.e., honeymoon bladder)
Spermicide use within past year (alone or with diaphragm)
Other risk factors: Infected renal calculi, low fluid intake, poor hygiene, catheterization
A sexually active female complains of new onset of dysuria, frequency, frequent urge to urinate, and nocturia. May also complain of suprapubic discomfort. Not associated with fever. Urine dipstick will show a moderate to large number of leukocytes and will be positive for nitrites. May show a few RBCs (due to inflammation), and be negative for ketones (unless fasting), and protein.
UA dipstick (midstream sample): Leukocyte positive (WBCs ≥10/mcL)
Nitrites: Negative or positive (E. coli converts urinary nitrate to nitrite)
Sometimes: Hematuria (>5 RBCs) and/or a few WBC casts
Urine C&S: Definitions
• UTI infection: 100,000 CFU/mL (or 105 CFU/mL) with pyuria
• Multiple bacteria: Contaminated sample
• Bacteriuria (with or without indwelling catheter): More than 100,000 CFU/mL
Urinary casts (tubular-shaped structures)
• RBC casts: Microscopic bleeding in the glomeruli; suspect glomerulonephritis (accompanied by edema, weight gain, dark cola-colored urine or hypertension)
• WBC casts: Due to inflammation; rule out pyelonephritis, interstitial nephritis
Healthy female patients aged 18 years to 65 years can have the “3-day” treatment regimen (these agents are used for 5–7 days for complicated UTIs). Routine urine C&S before and after treatment is not recommended for this population.
• Trimethoprim–sulfamethoxazole (Bactrim, Septra) BID × 3 days