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Cellular casts and renal patterns

17 min

Skills you'll build3 skills
  • Interpret an RBC cast as evidence of bleeding within the nephron
  • Distinguish the renal evidence of a WBC cast from free urinary leukocytes
  • Place renal injury in glomerulus or tubule from protein, heme, cells, and casts together

At the bench: Identify and report cells, casts, crystals, and other urine sediment findings · Reconcile urine appearance, chemistry, and microscopy findings

Try first

Try first

A fresh voided urine shows many red cells and two structures in which red cells are packed inside a matrix with parallel sides. The requisition notes that the patient is menstruating. What do the two structures mean?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Red cell casts

A red cell cast holds red cells inside a defined matrix. The cells entered the tubule upstream of where the cast set, most often through an injured glomerulus. A red cell cast is the strongest urine evidence of bleeding within the nephron, especially a glomerular process.1,2 Free red cells from the bladder or from menstrual blood cannot produce one. A loose clump of red cells without an outline is reported as red cells.1 Red cell casts are fragile, so they are sought at low power in a fresh specimen, often with phase contrast.1,3

White cell casts

A white cell cast holds leukocytes inside a matrix. It places the inflammation in the kidney, which free white cells do not. Pyelonephritis, interstitial nephritis and glomerular disease all produce white cell casts.1,2 The cast does not identify bacteria as the cause. The culture and the other findings do. Lobed nuclei identify neutrophils. Cells with one round nucleus inside a cast suggest renal tubular cells, and phase contrast or a stain confirms it.1

Reading the pattern

Protein, heme, cells, casts and concentration together localize the injury. A single finding does not.1,2

SiteSupporting pattern
Glomerular inflammationHematuria, acanthocytes or dysmorphic red cells, red cell casts, variable protein
Glomerular permeabilityMarked proteinuria, oval fat bodies, fatty casts
Acute tubular injuryRenal tubular cells and their casts, muddy brown granular casts, mild protein, poor concentration
Tubulointerstitial inflammationPyuria, white cell casts, mild protein, microscopic hematuria
Lower urinary tractPyuria, bacteria, hematuria, no renal casts

The laboratory reports the findings. The cause of a pattern is determined by further testing and the clinical evaluation.2

Follow the bench demonstration

Sources for this step
References
  1. Kouri TT, Hofmann W, Falbo R, et al. The EFLM European urinalysis guideline 2023. Clin Chem Lab Med. 2024;62(9):1653-1786. doi:10.1515/cclm-2024-0070
  2. Cavanaugh C, Perazella MA. Urine sediment examination in the diagnosis and management of kidney disease: Core Curriculum 2019. Am J Kidney Dis. 2019;73(2):258-272. doi:10.1053/j.ajkd.2018.07.012
  3. Strasinger SK, Di Lorenzo MS. Urinalysis and Body Fluids. 7th ed. F.A. Davis; 2021. ISBN 978-0-8036-7582-7. F.A. Davis catalog record.

Watch one

Read each decision and its explanation.

A fresh clean-catch urine comes from a 24-year-old man with dark urine two weeks after a sore throat. The report shows:

  • Strip: specific gravity 1.020, protein 2+, blood 3+, leukocyte esterase trace.
  • Sediment: red cells 20 to 50/HPF, many of them acanthocytes, and red cell casts 0 to 2/LPF.
  • Also: white cells 5 to 10/HPF, no white cell casts, rare squamous cells.

Where does the pattern place the injury?

  1. Check the specimen: it is fresh, and rare squamous cells indicate a good collection.

    A pattern can be interpreted only on a specimen that represents the patient.

  2. Read the chemistry: protein 2+ with blood 3+.

    Protein and heme together raise the question of a renal source.

  3. Read the red cells: many acanthocytes among 20 to 50 per field.

    Acanthocytes support glomerular bleeding, and red cell morphology is read on a fresh specimen.

  4. Read the casts: red cell casts are present.

    A red cell cast places the bleeding inside the nephron.

  5. Check the remaining findings: white cells are mildly increased, with no white cell casts.

    A few white cells can accompany glomerular inflammation, and white cell casts would suggest another site.

The protein, acanthocytes and red cell casts together fit glomerular inflammation. Report the findings. The cause is determined by further testing.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Hematuria with dysmorphic RBCs, RBC casts, and proteinuria most strongly supports which anatomic pattern?

Incorrect. Lower-tract inflammation lacks a renal cast pattern. RBC casts and dysmorphic RBCs place the bleeding in the kidney, particularly the glomerulus.

Correct. Dysmorphic RBCs and RBC casts place the bleeding in the glomerulus. Complement, disease-directed serology, and biopsy identify the cause.

Incorrect. Tubulointerstitial inflammation shows pyuria and white blood cell casts. RBC casts and dysmorphic RBCs point to the glomerulus.

Hint
  1. Consider which findings can come only from inside the kidney.
  2. Dysmorphic red cells and red cell casts point to the same part of the nephron.

Review Urine patterns and anatomic localization

Problem 2 of 3 · Hints available

Point to the cast that is the strongest evidence of bleeding within the nephron.

Four unlabeled urine sediment photomicrographs in two rows of two.

Image sources and licenses

  1. Top left
  2. Top right
  3. Bottom left
  4. Bottom right
Hint
  1. Look inside each cast at what it holds.
  2. Small, even, round cells without nuclei are red cells.
Show the answer

Red cell cast

The red cell cast holds red cells packed inside a matrix, which places the bleeding in the nephron. The tubular cell cast points to tubular injury, and hyaline and granular casts contain no red cells.

Review Casts

Problem 3 of 3 · On your own

A fresh urine shows many white cells, several casts packed with neutrophils, bacteria and a positive nitrite. The culture is pending. What does the white cell cast establish?

Casts form only inside the renal tubule. A white cell cast places the inflammation in the kidney.

Contamination adds squamous cells and organisms. It cannot put white cells inside a cast.

Interstitial nephritis and glomerular disease also produce white cell casts. The culture and the other findings identify the cause.

Called a WBC cast specific for bacterial pyelonephritis

A WBC cast localizes inflammation to the kidney. Pyelonephritis, interstitial nephritis, and glomerular disease can all produce WBC casts, so the cast does not show a bacterial cause. Culture and the other urine findings identify the cause.

The cast places the white cells inside the tubule. The bacteria and nitrite make infection likely, and the culture confirms it.

Review White blood cells

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Tobias Capper, 63, is in the intensive care unit after a long operation with low blood pressure.
  • His creatinine rose from 1.0 to 2.6 mg/dL in 48 hours.
  • A catheter urine is examined 30 minutes after collection.
  • Strip: specific gravity 1.010, protein 1+, blood 1+, leukocyte esterase negative.
  • Sediment: many muddy brown granular casts and renal tubular cells.
  • Several casts hold round cells, each with one round nucleus.
  • Red cells 3 to 5/HPF, about 1 in 20 an acanthocyte, plus one loose cluster of red cells with no outline.
Decision 1 of 3

How do you report the casts that hold round cells with one round nucleus?

Neutrophils have lobed nuclei, and the leukocyte esterase is negative. A single round nucleus fits renal tubular cells.

One round nucleus, free tubular cells in the same field and a negative esterase all fit tubular cells inside the matrix.

Red cells have no nucleus. These cells each have one.

Review Epithelial cells and lipid

Decision 2 of 3

How do you report the loose cluster of red cells?

The cluster has no matrix and no outline around it. Its cells are counted with the other red cells.

A cast needs a continuous matrix with parallel sides. Red cells lying close together without one form a clump.

The red cells are counted and reported. The report describes what was seen, and the source is interpreted from the whole pattern.

Review Casts

Decision 3 of 3

Which pattern do the findings fit?

A few acanthocytes among 3 to 5 red cells per field are one finding. With no red cell casts and only 1+ protein, the rest of the urinalysis indicates another site.

Localized renal injury from one isolated sediment finding

Localizing renal injury requires the combined pattern of protein, heme, cells, casts, and concentration. Placing injury in the glomerulus or tubule from one finding, such as a few dysmorphic RBCs, reports a localization the rest of the urinalysis may contradict.

That pattern includes pyuria and white cell casts. The cellular casts here hold tubular cells, and the esterase is negative.

The lower tract does not form casts, and this sediment contains many.

Muddy brown granular casts, tubular cells and their casts, mild protein and a specific gravity of 1.010 in a man with a rising creatinine form the tubular pattern.

Review Urine patterns and anatomic localization

The pattern rests on the tubular cells inside casts together with the muddy brown granular casts. A few acanthocytes do not change it. The report states each finding, and the pattern fits acute tubular injury.

For reference

Sources checked