Understanding Centrifugation for In-House Veterinary Urinalysis

Urinalysis is a routine part of veterinary medicine, but getting useful information from a urine sample involves more than running a simple test. A complete urinalysis can include evaluation of physical appearance and urine specific gravity, chemical testing, and microscopic examination of the urine sediment.¹

For practices performing urinalysis in-house, sediment preparation is one part of the process where centrifugation can have a significant effect on what is recovered and observed under the microscope. Sample volume, centrifugal force, spin time, residual volume, and resuspension technique all contribute to the final sediment preparation.

Understanding how these factors work together can help veterinary practices establish a more consistent approach to urine sediment examination.

What Is Included in a Complete Veterinary Urinalysis?

A complete urinalysis generally includes three areas of evaluation:

• Physical appearance and urine specific gravity
• Chemical testing
• Microscopic examination of urine sediment

Urine sediment examination is one part of the overall urinalysis rather than a stand-alone diagnostic test. It concentrates and examines formed elements that may not be fully characterized through gross appearance or chemical testing alone.¹

Depending on the sample and clinical context, sediment examination may include evaluation of red blood cells, white blood cells, epithelial cells, crystals, casts, and bacteria or other microorganisms.

Why Examine the Urine Sediment?

Urine sediment examination provides a closer look at formed elements present in the sample.

Red blood cells provide microscopic evidence of blood in a patient’s urine, while white blood cells are inflammatory cells that may be present in the sediment. Epithelial cells are shed from different portions of the urinary tract, and their type and quantity can help distinguish normal shedding or collection contamination from urinary tract irritation, inflammation, or renal tubular injury.¹

Crystals can also be identified and characterized by type and amount. Crystalluria may indicate a risk for kidney stones or urolithiasis, although crystals can also be incidental. Their significance should be considered alongside urine pH, clinical findings, and sample temperature.¹

Casts form within the renal tubules. Increased numbers or cellular casts can indicate renal tubular injury or inflammation. Because casts can be sparse and fragile, their detection can be particularly sensitive to sample preparation and examination technique.¹

Bacteria may also be detected during sediment analysis. The ASVCP recommends examining a stained sediment smear when bacteriuria is suspected, although quantitative urine culture is recommended for confirming significant bacteriuria or infection.¹

Sediment examination is commonly performed when urinary signs are present or when abnormalities elsewhere in the urinalysis warrant further examination. At the same time, the way sediment examination is performed can vary between veterinary practices and technicians. A survey by Gibbs et al. documented substantial variation in urine sediment preparation methods among veterinary practitioners. Nearly half of respondents used less than 3mL of urine for sediment preparation, and 63% retained less than the recommended 0.5mL of supernatant for resuspension²

Why Urine Sediment Preparation Needs to Be Standardized

Centrifugation concentrates formed elements into a pellet for microscopic examination. What happens during that process affects the concentration of material ultimately examined.

Changing the starting volume, centrifugal force, centrifugation time, amount of supernatant retained, or volume used for resuspension can change the final sediment concentration.¹

Sediment preparation also depends on specimen handling. The ASVCP recommends examining urine ideally within 30 minutes of collection. Delayed analysis and storage conditions can alter formed elements, and crystals may precipitate after collection.¹

One example of a standardized approach uses 5 mL of urine when sufficient sample is available, centrifuged at 400 xg for 5 minutes. The method also uses a consistent residual volume, with 0.5 mL recommended, followed by gentle resuspension and a consistent slide volume and coverslip.¹

The specific preparation method used by a practice should be applied consistently from sample to sample. Using the same steps can make results more comparable between technicians and over time.

RCF vs. RPM: Why the Numbers Are Not Interchangeable

One of the most important things to understand when centrifuging urine is the difference between RCF and RPM.

RCF, or relative centrifugal force, describes the force applied to the sample. RPM, or rotations per minute, describes how quickly the rotor is spinning. Because the rotor radius affects the RCF produced at a given speed, two centrifuges can produce different RCF values at the same RPM.

There is no universal RPM value that corresponds to 400 xg.

When moving a urine sediment method from one centrifuge to another, the required RCF should be reproduced rather than simply copying the RPM setting from the original centrifuge.

For example, Drucker Diagnostics’ centrifuges designed for urine processing can use different RPM settings to produce the same RCF. The TrueBond veterinary medicine centrifuge and the TrueBond Set 3 use a urine preset while the TrueBond TriFLEX uses 1,650 RPM to produce the same 500 xg for 5 minutes. The difference in RPM reflects the different rotor geometry of the centrifuges.

If you need to calculate the RCF on your unit, you can use this formula:

RCF = 1.118 × 10⁻⁵ × r × N²

where r is the reference radius in millimeters and N is the speed in RPM.

Conventional-Volume and Small-Volume Sediment Methods

Not every veterinary urine sample provides the same volume, and different sediment preparation methods can be used when sample volume is limited.

Chase et al. compared several urine sediment preparation methods, including a conventional-volume method using 5 mL of urine at 390 xg for 5 minutes and a small-volume, high-force method using 1.5 mL at 3,900 xg for 45 seconds.³

The study found that RBC and WBC counts differed between sediment preparation methods. The small-volume, high-force method produced approximately half the RBC and WBC counts observed across the 10 microscopic fields than the conventional 390 xg, 5-minute method. RBC morphology also differed.³

 These findings support treating the small-volume, high-force method as a distinct preparation method rather than simply a faster version of a conventional sediment preparation.

Chase et al. used quality-control material and canine RBC suspensions rather than clinical urine specimens. The study demonstrated differences between preparation methods but did not establish diagnostic performance in clinical urinalysis or validate a specific commercial centrifuge.³

Why Both Force and Time Matter

More RCF does not necessarily mean a more useful urine sediment.

The ASVCP guidelines caution that excessive centrifugal force or centrifugation time can damage casts and cellular elements.¹ The findings from Chase et al. provide direct evidence that changing the sediment preparation method can affect cell recovery and RBC morphology.³

Cast detection can be  particularly difficult even when specimens are prepared appropriately. Vasilatis et al. found only moderate agreement between automated sediment analysis and manual microscopy for detection of casts in canine urine, illustrating how examination methods can influence sediment findings.⁴

For that reason, it is not useful to assign one centrifugal force as the universally correct setting for urine sediment. RCF needs to be considered as part of the complete preparation method, including sample volume, centrifugation time, residual volume, and resuspension.

Matching the Centrifuge to the Urine Workflow

The centrifuge should be selected based on the urine sediment method the practice intends to use, rather than simply choosing the model with the highest maximum RPM.

Start by considering the specimen volume and sediment preparation method. Then confirm the required RCF, centrifugation time, tube size, rotor or carrier configuration, and sample capacity.

For conventional-volume urine processing, the TrueBond and Set 3 provide a urine preset of 500 xg for 5 minutes and accommodate tubes up to 15 mL. The two models use different control interfaces.

The TriFLEX also provides a 500 xg, 5-minute urine setting, with a horizontal rotor and broader tube-size capacity.

For practices using a small-volume urine method, the TrueBond Duet uses 1.5–2 mL tubes and has a urine preset of 3,900 xg for 45 seconds. That volume, RCF, and time values correspond to the small-volume, high-force method evaluated by Chase et al.³

The goal is not to select a centrifuge based on maximum RPM alone. Instead, match the centrifuge to the sediment preparation method the practice intends to use.

Standardizing Urine Sediment Preparation In-House

Consistent urine sediment preparation starts with defining the method used by the practice. That includes establishing the starting sample volume, RCF, centrifugation time, residual volume, and resuspension technique.

Once those parameters are established, using the same preparation steps for each sample can help make sediment examination more consistent between technicians and over time.¹

When a practice changes centrifuges, the existing RPM setting should not simply be transferred to the new model. Confirm the required RCF and determine the appropriate RPM for the new centrifuge based on its rotor characteristics.

A standardized approach gives the veterinary team a defined process to follow and makes the centrifugation portion of in-house urinalysis more consistent from sample to sample.

References

1. Arnold JE, Camus MS, Freeman KP, et al. ASVCP Guidelines: Principles of Quality Assurance and Standards for Veterinary Clinical Pathology (version 3.0). Veterinary Clinical Pathology. 2019;48(4):542–618. doi:10.1111/vcp.12810.
2. Gibbs NH, Heseltine JC, Rishniw M, Nabity MB. Use of urinalysis during baseline diagnostics in dogs and cats: an open survey. Journal of Small Animal Practice. 2023;64:88–95.
3. Chase J, Hammond J, Bilbrough G, DeNicola DB. Urine sediment examination: Potential impact of red and white blood cell counts using different sediment methods. Veterinary Clinical Pathology. 2018;47(4):608–616. doi:10.1111/vcp.12674.
4. Vasilatis DM, Cowgill LD, Farace G, et al. Comparison of IDEXX SediVue Dx urine sediment analyzer to manual microscopy for detection of casts in canine urine. Journal of Veterinary Internal Medicine. 2021;35(3):1439–1447. doi:10.1111/jvim.16090.