Skip to content

Add your earlier progress to your account?

From before you signed in, this browser has:

Which plan do you want to keep?

You answered the plan questions before you signed in, and those answers don't match the plan saved on your account.

SettingSaved planNew answers

Study progress is waiting to be saved

Oval fat bodies, yeast and artifacts

15 min

Skills you'll build3 skills
  • Recognize an oval fat body and confirm its lipid by polarization or fat stain
  • Distinguish yeast from red cells using budding and related features
  • Distinguish contaminants and pseudocasts from true urine sediment

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

Try first

Try first

Under polarized light, a urine sediment shows round particles with a bright Maltese cross. They lie free in the field, vary in size and have a small central dimple. The protein pad is negative. What are they?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Oval fat bodies

An oval fat body is a renal tubular cell or a macrophage filled with refractile lipid droplets. Cholesterol esters in the droplets show a Maltese cross under polarized light. Triglycerides show no cross and take up a fat stain such as Oil Red O or Sudan III.1,3 Oval fat bodies and fatty casts support lipiduria with heavy protein loss, so marked proteinuria is usually present as well.1,2

Confirm an oval fat body in three steps:

  1. The droplets are inside a cell.
  2. Polarized light shows Maltese crosses, or a fat stain colors the droplets.
  3. The protein result fits heavy protein loss.

Yeast and red cells

Yeast are refractile oval forms that vary in size. They bud, and some form pseudohyphae. Red cells are smooth discs of even size with no buds. Dilute acetic acid on a separate portion lyses red cells and leaves yeast intact.3 A negative blood pad with many apparent red cells is a reason to look again for budding.1 Yeast in urine often reflect colonization, and the urine findings cannot distinguish colonization from infection.4

Artifacts

Artifacts are often very refractile or lie in a different focal plane from the sediment. Fibers and hair vary in width, twist, fray at the ends and polarize strongly. They have no continuous cast matrix. Fine focus moves an air bubble or an oil droplet sharply out of view.1,2

ParticleMaltese crossIdentifying features
Oval fat bodyYes, with cholesterol estersDroplets inside a cell, heavy proteinuria
Triglyceride dropletNoTakes up a fat stain
Starch granuleYesFree, central dimple, varied size
Budding yeastNoBuds and pseudohyphae
Red cellNoEven disc, lysed by acetic acid

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. Pappas PG, Kauffman CA, Andes DR, et al. Clinical practice guideline for the management of candidiasis: 2016 update by the Infectious Diseases Society of America. Clin Infect Dis. 2016;62(4):e1-e50. doi:10.1093/cid/civ933

Watch one

Read each decision and its explanation.

A clean-catch urine comes from an outpatient with diabetes. The strip shows glucose 1,000 mg/dL, blood negative and protein negative. At high power the field is full of small round refractile forms about the size of red cells. What are they?

  1. Compare with the strip first: a negative blood pad with a field full of apparent red cells does not agree.

    The strip and the sediment come from one specimen, so they should agree.

  2. Look at shape and size: the forms are oval and uneven in size.

    Red cells are even discs, and yeast vary in size and are often oval.

  3. Focus finely through the forms: many have a small bud attached, and a few chains of pseudohyphae are present.

    Budding is the feature a red cell never shows.

  4. When the identification is uncertain, add dilute acetic acid to a separate portion: the forms stay intact.

    Acetic acid lyses red cells and leaves yeast intact.

The forms are budding yeast. Report yeast in the procedure's category, with no red cells from these forms.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Point to the structure that is left out of the patient's report.

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. A structure that came from outside the patient is left out.
  2. Look for twists, uneven width and frayed ends without a cast matrix.
Show the answer

Contaminant fiber

The contaminant fiber twists, frays at the ends and has no cast matrix. It is a contaminant and stays out of the report. Mucus comes from the patient and is reported in the procedure's category. The two casts are patient findings.

Review Contaminants and artifacts

Problem 2 of 3 · Hints available

Point to the particles that show a Maltese cross under polarized light and contain no lipid.

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. The particle comes from outside the patient.
  2. Look for particles that lie free, outside any cell, with a central dimple.
Show the answer

Starch granules

Starch granules show a Maltese cross and contain no lipid. They lie free and have a central dimple. An oval fat body has its crossed droplets inside a cell.

Review Epithelial cells and lipid

Problem 3 of 3 · On your own

A urine with protein 4+ shows round cells, each with one nucleus and packed with refractile droplets. Under polarized light the droplets show no Maltese cross. What do you do next?

Starch lies free and shows a Maltese cross. These droplets sit inside cells and show none.

Triglycerides show no cross under polarized light. A negative polarization result does not exclude neutral lipid.

White cells are not packed with refractile droplets, and a neutrophil's nucleus is lobed.

Oil Red O or Sudan III colors neutral lipid that does not polarize. Stained droplets inside the cells confirm oval fat bodies, which fit the heavy proteinuria.

Review Epithelial cells and lipid

Use it

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

  • A student's worksheet is waiting for your verification.
  • The specimen is a clean-catch urine from Mateo Reagan, 58, a regular at the diabetes clinic.
  • Strip: glucose 500 mg/dL, blood negative, protein negative.
  • The student recorded red cells 25 to 50/HPF and "a few waxy casts".
  • You recheck the sediment. The round forms vary in size, and many carry buds.
  • The "casts" are long bright strands that twist, fray at the ends and polarize strongly.
Decision 1 of 3

How are the round forms reported?

Buds, uneven size and a negative blood pad all indicate yeast.

A negative blood pad does not fit 25 to 50 red cells per field, and red cells never bud.

Called every round RBC-sized particle a red cell

Yeast, oil droplets, and starch mimic red cells. Counting budding yeast as red cells reports hematuria that the blood pad may not support and misses the yeast. Budding, pseudohyphae, and fine focus separate them.

The forms you rechecked are one population with buds. Nothing in the field or on the strip shows red cells.

Review Microorganisms, sperm, and mucus

Decision 2 of 3

What happens to the "waxy casts"?

A waxy cast has parallel sides, blunt ends and a continuous matrix. Twisting, fraying and strong polarization describe a fiber.

Reported an elongated fiber as a cast

Fibers and hair have irregular width, twists, frayed ends, and strong polarization, and they lack a continuous matrix. Reporting one as a cast adds a renal finding the specimen does not have.

The strands have the features of a fiber and no matrix. A fiber is a contaminant and is left out of the report.

Mucus is pale with low refraction and does not polarize strongly. These strands are bright and strongly birefringent.

Review Contaminants and artifacts

Decision 3 of 3

What do you do with the corrected worksheet?

The specimen is fresh and suitable. The errors were in reading the sediment, and the recheck has corrected them.

The student's red cells and casts are incorrect results. A comment does not correct results known to be wrong.

The recheck agrees with the strip and gives the correct sediment. The corrected results are ready to release.

Review Specimen integrity and interference

The discrepancy is in the negative blood pad. It did not fit 25 to 50 red cells per field, and a closer look showed buds on the forms. The strands had no matrix and polarized strongly. The corrected report lists yeast and no red cells or casts.

For reference

Sources checked