Accurate Kidney Stone Analysis [Hot Topic]

David Murray, M.D., Ph.D.

The composition of a kidney stone can be an important diagnostic detail. When kidney stone specimens are submitted in less than optimal conditions, it delays analysis and may actually impact the analytic outcome. Proper preparation prior to submitting the specimen can greatly improve both turnaround time and accuracy.

Presenter and Credentials:
David Murray, M.D., Ph.D., Assistant Professor of Laboratory Medicine and Pathology, Consultant in the Division of Clinical Biochemistry, and Co-Director of the Metals Laboratory at Mayo Clinic in Rochester, Minnesota


Kidney stones are......Thank you for the introduction. I have nothing to disclose. I am David Murray, one of the co-directors from Metals testing lab at Mayo Clinic. This lab oversees the kidney stone analysis.

At Mayo Medical Laboratories, we have a long history of providing accurate and rapid kidney stone analysis. Our lab analyzes more than 60,000 kidney stones per year. I am here to update you on the proper procedures for collecting and processing kidney stones in order to provide accurate, cost-effective analysis of patients’ kidney stones in a timely manner.
Kidney stones are quite common in the general population. It is estimated that 1 out of 10 people will develop a kidney stone during their lifetime. Epidemiology indicates that the incidence of kidney stones is increasing. Interestingly, the highest increase is in the pediatric population. The causes of kidney stones are quite complex, resulting from a combination of genetic, dietary, and life-style factors. From the lab’s point of view, we can aid in determining the cause by careful examination of the urine and stone constituents. Information about the stone composition is used in both medical and surgical interventions. If you ever had a kidney stone, you realize the trauma and pain they can cause. I cannot stress enough that the proper handling and labeling of kidney stones samples is critical to accurate stone composition analysis. Given the irretrievable nature of the stone, we have one chance of obtaining an accurate compositional analysis. Hence, we must all take the upmost precautions to assure that the information is not lost. Each step of the analysis process is key to assure the accuracy of the data. Thus, kidney stones should be given the same procedural policies as surgical specimens to assure that proper identification and analysis results are maintained. Information about the stone can easily be lost if the specimens are not labeled or are labeled incorrectly.

First Step: Proper Collection

Proper analysis of stone collections requires proper collection. Although it is difficult for a patient to know exactly when a stone will pass, the odds of collecting the stone are increased when patients properly filter their urine. Our lab has seen many unique and sometimes odd ways that patients will try to collect their stone. To help our clients simplify the process, Mayo Medical Laboratories offers a collection kit with a filter, vial, and detailed instructions for patients and lab personnel on how to collect kidney stones. Unfortunately, not many of our clients take advantage of these kits and we estimate that only about 25% of the kidney stone samples arrive in our kit containers.

Clients can order kits by logging into, selecting the Customer Service page, and from there, selecting the Supplies page. Alternatively, orders may be placed by calling Customer Service at 800-533-1710.

The stone must be clean and dry for proper analysis

Stones need to be clean and dry before analysis. The analytical method by which we determine the composition absolutely requires the stone to be clean and dry. Excess water in the stone will cause a major interference in the compositional analysis and can totally obscure our results. Excess blood and tissue can cause a false-determination that the stone is proteinaceous and mask the true composition of the stone. Therefore, we require that all stones be clean and dry before analysis.

Currently, about 30% of the stones arrive like the stones in the left picture. This forces our lab to spend extra time drying the samples before analysis. This results in additional handling and delayed turnaround time. Currently, our lab does not pass on the extra cost to our clients, but this does increase the overall cost of performing the test.

Analysis Method

In the lab we use infrared spectroscopy to determine the composition of the stone. A kidney stone is crushed into a powder and placed on top of a crystal. An infrared beam is passed through the crystal, interacting with the powdered kidney stone sample. During this interaction, the molecular bonds within the stone absorb some of the radiation giving a unique spectra as we will see on the next slide.

Example of a COM Control Spectrum

This is an example of a very common stone composition: calcium oxalate monohydrate. The infrared spectral analysis of the stone produces a unique pattern of absorption, which we call the finger print, which makes the composition identification relatively simple.

Example of a Wet Stone Spectrum

This is an example of a wet stone. Water has very strong UV absorbance and the presence of water is masking that unique finger print of the stone composition.

What should your lab do if you received a wet or dirty stone from the patient?

Although the answer to this question may seem rather obvious, there are some special considerations to keep in mind when drying stones. Stones containing obvious blood should be rinsed with deionized water prior to drying. Drying wet stones must not be done at too high of a temperature. For example, struvite stones, which are typically the yellow-to-brown stones, are composed of magnesium ammonium phosphate, if we heated these stones too aggressively we can eliminate ammonia, thereby changing the composition. We recommend placing the stone on a filter paper at room temperature until the stone is dry. Once dry, the stone can be placed in the 10-mL tube provided with our collection kit.


In summary, proper specimen handling is critical for accurate determination of kidney stone composition. This includes how you process your patients’ specimens prior to submitting them to Mayo Medical Laboratories. By following the Kidney Stone Analysis Patient Collection Instructions and the Kidney Stone Analysis Packaging Instructions included in the MML kit, you can improve the quality of the test results you receive. Thank you for your attention.


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This post was developed by our Education and Technical Publications Team.