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Diagnostic evaluation of the kidney and urinary tract

Last updated: March 26, 2021

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This article covers important clinical findings of urological and renal conditions, including changes in micturition (e.g., dysuria, anuria) and changes in urine (e.g., hematuria, proteinuria) as well as the routine diagnostics for the initial evaluation of patients with urological and renal symptoms, including urinalysis and renal function testing. Urinalysis helps to evaluate urinary abnormalities and involves gross assessment of urine, dipstick, and microscopy of urine sediment. Dipstick is a diagnostic tool consisting of a urine test strip that allows for quick assessment of potentially pathological changes of various parameters (e.g., pH, glucose, protein). Urine sediment allows for microscopic detection of cells, urinary casts, urinary crystals, bacteria, and yeast in a urine sample. Renal function testing involves a panel of parameters to assess for renal dysfunction. Inulin clearance and creatinine clearance provide the most accurate calculation of the GFR while serum creatinine can be used together with demographic data to provide an estimated GFR. The BUN/creatinine ratio and fractional excretion of sodium can be used to evaluate the cause of acute kidney injury. The findings of these tests may be diagnostic or provide guidance for further diagnostic evaluation (e.g., renal biopsy, imaging)

For more on imaging and urodynamic tests, see “Diagnostic investigations in urology.” For more on urine culture, see “Laboratory tests” under diagnostics in “Urinary tract infections.” Current NBME laboratory reference values can be found under “Tips and Links” below.”

Clinical examination

Acute appendicitis should be differentiated from right-sided renal colic. Findings that suggest appendicitis include nausea, fever, and pain at McBurney point (see appendicitis signs).

Changes in micturition

Overview of micturition changes
Definition Common diagnoses
Quality of micturition
Dysuria
  • Pain or discomfort during micturition
Pollakiuria
Nocturia
  • Waking up to urinate ≥ 2 times at night
Quantity of urine excretion
Polyuria
  • Urine output > 3 L/day in adults
Oliguria
  • Urine output < 400 mL/day [1]
Anuria
  • Urine output < 50 mL/day

Changes in urine

Overview of urine changes
Definition Common diagnoses
Isosthenuria
Glycosuria
  • Glucose in the urine
  • Occurs when blood glucose levels exceed 180 mg/dL (renal threshold for reabsorption of glucose)
Ketonuria
Proteinuria
  • > 150 mg protein/day in the urine
Bacteriuria

Pyuria

Hematuria
Hemoglobinuria
Myoglobinuria

Urinalysis involves the gross examination of urine, chemical evaluation using urine dipstick, and microscopic assessment of urine sediment. Further tests include urine culture and urinary electrolyte levels. Indications for urinalysis include renal, urinary, and metabolic conditions.

Gross urine assessment

Certain drugs (e.g., rifampin, phenazopyridine), foods (e.g., beetroot), and types of porphyria cause red discoloration of urine.

Urine dipstick

A diagnostic tool consisting of a urine test strip that allows for quick assessment of potentially pathological changes of various parameters.

A urine dipstick cannot differentiate between hematuria, hemoglobinuria, or myoglobinuria!. Therefore, every positive test result for heme must be confirmed with the presence of RBCs on microscopy.

Urine sediment

Urinary casts [2]
Structure Microscopy Interpretation
Hyaline casts
  • Composed of Tamm-Horsfall mucoproteins, which are secreted by renal tubular cells in order to prevent urinary tract infections
  • Homogeneous, transparent, and eosinophilic
Granular casts
  • Caused by degeneration of cellular casts
  • Composed of a hyaline matrix with cellular debris
Muddy brown casts
Hemoglobin casts
  • Result from degeneration of RBCs within the cast matrix.
  • Cells are no longer visible, but hemoglobin pigment remains, giving the casts an orange-yellow or red-brown color.
Fatty casts
  • Formed from congregated fat droplets and “oval fat bodies with a protein matrix
  • Round fat droplets with variable size
  • Maltese cross” appearance within fat droplets under polarized light
Renal tubular epithelial cell casts
White blood cell casts
  • Accumulated WBCs within a protein matrix
Red blood cell casts
  • Accumulated RBCs in a mucoprotein matrix
Waxy casts
  • Degenerating granular cast
  • Homogeneous, sharp indentations
  • Edges that appear more distinct and dark
Broad casts
  • Wide casts
  • Formed in dilated tubules or collecting ducts with extremely low flow
  • Wider than other casts

Further laboratory tests

Overview

Glomerular filtration rate (GFR) [3][4]

  • Defined as the volume of primary urine that is filtrated by the kidneys over a certain amount of time per standardized body surface area (1.73 m2)
  • Normal GFR: ≥ 90 mL/min/1.73m2
  • GFR is ∼ 120 mL/min/1.73m2 in young adults, decreases with age, and varies considerably between males and females
  • After the age of 29, a physiological decrease in the GFR of about 10 mL/min/1.73m2 occurs every 10 years.
  • GFR can be calculated or estimated using various methods (e.g., estimated GFR).

Serum creatinine

Serum creatinine levels do not start rising until the GFR is reduced by approx. 50%.

Creatinine clearance

Creatinine clearance can be used to approximate the GFR.

Estimated GFR (eGFR)

  • Calculated using serum creatinine concentration and demographic data
  • Several prediction equations can be used in clinical practice.
    • Cockcroft-Gault equation: creatinine clearance = [(140 – age) x weight (kg) x constant] / Constant = serum creatinine (mmol/L)
    • Modification of Diet in Renal Disease (MDRD) study equation
    • Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation

Serum cystatin C

  • A more precise indicator of the GFR than serum creatinine
  • Cystatin C is a small protein that inhibits cysteine proteinases and is produced by all nucleated cells.
  • Analysis is more complex and expensive; therefore, not routinely ordered.
  • Lower “blind range” than serum creatinine: used particularly when urine sampling is not feasible (e.g., in infants)

Blood urea nitrogen (BUN)

BUN/creatinine ratio

  • Can help diagnose the underlying cause in acute kidney injury
    • 10:1–20:1 can be normal or may indicate a postrenal cause.
    • ≥ 20:1 indicates prerenal cause: Urea reabsorption is increased, which is typical in patients with dehydration or hypoperfusion.
    • ≤ 15:1 indicates intrarenal cause: Renal damage causes decreased urea reabsorption.

Fractional excretion of sodium (FeNa)

  • Definition: percentage of the glomerular filtered sodium (NaFiltered) that is eventually excreted in the urine (NaExcreted)
  • Usage
  • Calculation
    • Filtered Na = NaPlasma x GFR
    • Excretion rate of Na = NaUrine x V
    • FeNa = NaExcreted / NaFiltered
    • FeNa = (NaUrine x V) / (NaPlasma x GFR)
    • FeNa = (NaUrine x V) / (NaPlasmax [CreaUrine x V / CreaPlasma])
    • FeNa = (NaUrine x CreaPlasma) / (NaPlasmax CreaUrine)
  • Interpretation
    • In acute kidney injury
      • Low FeNa (< 1%): indicates a prerenal cause (renal hypoperfusion)
      • High FeNa (> 2%): indicates an intrarenal etiology (e.g., acute tubular necrosis)
      • Inconclusive FeNa (1–2%): can be seen with either disorder
    • In hypotonic hyponatremia
      • Low FeNa (< 1%): extrarenal cause
      • High FeNa (> 1%): renal cause

Additional blood tests

Renal parameter changes

Overview of renal parameters
Laboratory parameter

Causes of increased values

Causes of decreased values

Creatinine

  • Low muscle mass
  • Underweight individuals
  • Pregnancy
Cystatin C
Blood urea nitrogen (BUN)
Uric acid

Overview

Indications

Contraindications

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  1. Klahr S, Miller SB. Acute Oliguria. N Engl J Med. 1998; 338 (10): p.671-675. doi: 10.1056/nejm199803053381007 . | Open in Read by QxMD
  2. Urinalysis. https://library.med.utah.edu/WebPath/TUTORIAL/URINE/URINE.html. Updated: January 1, 2017. Accessed: April 16, 2018.
  3. Redal-Baigorri B, Rasmussen K, Heaf JG. Indexing glomerular filtration rate to body Surface area: clinical consequences. J Clin Lab Anal. 2013; 28 (2): p.83-90. doi: 10.1002/jcla.21648 . | Open in Read by QxMD
  4. GFR. https://www.kidney.org/kidneydisease/siemens_hcp_gfr. . Accessed: May 31, 2018.
  5. Lin Y, Bansal N, Vittinghoff E, Go AS, Hsu C. Determinants of the creatinine clearance to glomerular filtration rate ratio in patients with chronic kidney disease: a cross-sectional study. BMC Nephrol. 2013; 14 (1). doi: 10.1186/1471-2369-14-268 . | Open in Read by QxMD
  6. Fischbach FT, Dunning MB. A Manual of Laboratory and Diagnostic Tests. Lippincott Williams & Wilkins ; 2009