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How casts form

15 min

Skills you'll build3 skills
  • Explain where and how urinary casts form
  • Distinguish a true cast matrix from a free aggregate of cells
  • Tell a few physiologic hyaline casts from the increased numbers seen in renal disease

At the bench: Identify and report cells, casts, crystals, and other urine sediment findings

Try first

Try first

A red cell cast is found in a urine sediment. Where in the urinary tract did the cast take its shape?

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.

How a cast forms

Cells of the thick ascending limb secrete uromodulin, also called Tamm-Horsfall protein. In the distal tubule and collecting duct, its fibrils gel into a cylinder that fills the lumen. Slow urine flow, acidic pH and concentrated urine help it form. Cells, protein, fat, pigment, organisms and crystals in the lumen are trapped as it sets.1,2 A cast therefore places its contents inside the kidney. A broad cast formed in a widened tubule or collecting duct.1

Telling a cast from a look-alike

A true cast has a continuous matrix with parallel sides and rounded or broken ends. Its contents sit inside that outline.1,2 Several structures can be mistaken for a cast:

  • A free clump of cells has no outline around it.
  • Mucus forms long, pale ribbons of uneven width with tapered or frayed ends.
  • A fiber twists, frays and polarizes strongly.
  • Crystals can line up into a clump, and their separate edges stay visible.

Scan for casts at low power over at least 10 fields, then identify the contents at high power.1

FeatureTrue castCell clumpMucus thread
OutlineContinuous matrixNoneRibbon without a defined edge
SidesParallelIrregularUneven width
EndsRounded or brokenIrregularTapered or frayed

Hyaline casts

A hyaline cast is matrix without inclusions. It is colorless and has a refractive index close to urine, so it is seen with reduced light or phase contrast.1,3 A few appear in healthy people after exercise, with dehydration or in concentrated urine. The ASCP examination reference is 0 to 2 per low-power field.4 Increased numbers can accompany renal disease, usually with protein and other casts.1,2

Casts dissolve in dilute alkaline urine and are lost as a specimen stands. Urine is examined within 2 hours of collection unless it is refrigerated or preserved.1,5

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.
  4. American Society for Clinical Pathology Board of Certification. Medical Laboratory Scientist, MLS(ASCP) and MLS(ASCPi) Examination Content Guideline. Revised June 9, 2026. Accessed September 27, 2026. ASCP BOC examination content guideline
  5. College of American Pathologists. Urinalysis Checklist. December 9, 2025 ed. Requirements URN.22300 and URN.22400. Applies to CAP-accredited laboratories. Accessed September 27, 2026. CAP accreditation checklists.

Watch one

Read each decision and its explanation.

A 19-year-old cross-country runner gives a urine 30 minutes after a race, and it reaches the laboratory in 40 minutes. The strip shows specific gravity 1.030, protein trace and blood negative. The hyaline casts counted in 10 low-power fields are 2, 1, 0, 3, 1, 2, 1, 0, 2 and 1. How do you interpret the casts?

  1. Confirm the lighting: the count was made with reduced light at low power.

    Faint hyaline casts disappear under full light.

  2. Average the counts: 13 casts ÷ 10 fields = 1.3 per low-power field.

    Casts are reported as the average over the number of fields the procedure specifies.

  3. Compare with the reference: 1.3 per field is within 0 to 2.

    The reference gives the expected range for a healthy person.

  4. Note the circumstances: the specimen was collected right after a race and is highly concentrated.

    Exercise and concentrated urine both increase hyaline casts.

  5. Check the other findings: there are no cellular, granular or waxy casts, and protein is trace.

    In renal disease protein, blood or other casts usually accompany the hyaline casts.

The average is 1.3 hyaline casts per low-power field. Report it in the procedure's category, with no comment about renal disease.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Which observation best distinguishes an RBC cast from a free clump of RBCs?

Round RBCs can occur freely in urine and inside a cast. The matrix establishes the cast structure.

Both a clump and a cast can contain many RBCs. Look for the surrounding matrix.

A defined matrix surrounds the embedded cells. A free RBC clump lacks that cast outline.

Hint
  1. A clump and a cast can contain the same cells.
  2. Look at what surrounds the cells.

Review Casts

Problem 2 of 3 · Hints available

Of these three long structures, point to the one with a true cast matrix.

Three unlabeled urine sediment photomicrographs, two above and one below.

Image sources and licenses

  1. Top left
  2. Top right
  3. Bottom
Hint
  1. Follow each structure from end to end.
  2. Check which one keeps an even width with parallel sides.
Show the answer

Granular cast

The granular cast has a continuous matrix with parallel sides and granules inside it. The mucus thread is a pale ribbon of uneven width. The fiber twists and tapers to frayed ends.

Review Casts

Problem 3 of 3 · On your own

A fresh urine comes from a hospitalized woman whose creatinine is rising. Protein is 2+. The sediment averages 8 hyaline casts and 3 granular casts per low-power field. How do you interpret the hyaline casts?

A few hyaline casts are physiologic. Eight per field is well above 0 to 2, and protein and granular casts are present.

The number is above the reference, the specimen is fresh, and protein and granular casts are present. The pattern fits renal disease.

Mucus has uneven width and tapered ends. A hyaline cast has parallel sides and rounded ends, and each is counted separately.

Casts dissolve in delayed, dilute alkaline urine. This specimen is fresh, so the count stands.

Review Casts

Use it

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

  • A urine from Rosalind Blank, 71, on a medical floor, was collected at 07:30.
  • It reached the laboratory at 14:10, unrefrigerated and without preservative.
  • Strip: pH 8.0, specific gravity 1.005, protein 1+.
  • Sediment: rare hyaline casts, many bacteria, and many long pale ribbons with tapered ends and uneven width.
Decision 1 of 3

What has the delay most likely done to the casts in this urine?

The cast matrix sets in acidic, concentrated urine. In dilute alkaline urine that stood for hours, casts dissolve.

Casts form inside the distal tubule and collecting duct. They do not form in a container.

Casts are among the first elements lost in a delayed specimen, especially in dilute alkaline urine.

Review Changes during storage

Decision 2 of 3

What are the long pale ribbons?

A hyaline cast has parallel sides and rounded ends. Uneven width and tapered ends describe mucus.

Fibers are bright and polarize strongly. These ribbons are pale.

Mucus forms long pale ribbons with tapered ends and uneven width, and it has no defined matrix.

Review Casts

Decision 3 of 3

What do you do with the microscopy?

The casts may have dissolved during the 6 hours 40 minutes at room temperature. A low count from this specimen cannot show how many casts were present when the urine was passed.

The comment does not correct the count. Rare hyaline casts could still be read as her actual finding.

The specimen is past the 2-hour limit without refrigeration or preservative. Only a fresh urine shows her actual sediment.

Review Changes during storage

The decision rests on the 6 hours 40 minutes at room temperature in dilute alkaline urine. Casts dissolve in those conditions, so the rare hyaline casts cannot be interpreted as a normal finding. The ribbons are mucus, and a fresh specimen is needed for a reportable sediment.

For reference

Sources checked