How to collect a 24-hour urine sample for oxalate measurement?

From the Guidelines

To collect a 24-hour urine sample for oxalate testing, the sample should be collected into acid or acidified within 24 hours after collection to achieve a pH of <2, as recommended by the most recent and highest quality study 1. The collection process should start by obtaining a special collection container from a healthcare provider or laboratory, and begin the collection by urinating into the toilet first thing in the morning and discarding this urine, then note this time as the start time. For the next 24 hours, collect all urine in the provided container, which typically contains a preservative to maintain sample integrity. Key considerations for the collection process include:

• Keeping the container refrigerated or in a cool place between uses to prevent degradation of the sample
• Consuming a normal diet during collection unless instructed otherwise, but avoiding excessive oxalate-rich foods like spinach, rhubarb, and chocolate, which could artificially elevate results
• Drinking normal amounts of fluid to ensure accurate representation of oxalate levels
• Completing the collection at the same time the next morning by adding the final urination to the container
• Returning the entire sample to the laboratory as soon as possible, along with any paperwork noting the exact collection times, as delayed processing can lead to microbial growth and degradation of biomolecules 1. It is also important to note that acidification of the sample can be done in the laboratory within 24 hours, provided the sample is kept at 4°C after collection 1. The goal of this test is to diagnose conditions like kidney stones, as oxalate is a compound that can combine with calcium to form the most common type of kidney stones, and accurate collection is essential for proper diagnosis and treatment planning.

From the Research

Collecting 24-Hour Urine for Oxalate

To collect 24-hour urine for oxalate, the following steps can be considered:

• The collection process typically involves gathering all urine produced over a 24-hour period, usually starting from the second void of the day and ending with the first void of the next day 2
• Patients should be instructed to discard the first void and then collect all subsequent voids for the next 24 hours, including the first void of the next day
• The urine should be stored in a container with a preservative to prevent bacterial growth and degradation of the oxalate
• The patient should be advised to avoid certain foods and substances that may affect oxalate levels, such as vitamin C supplements, which have been shown to increase urinary oxalate excretion 3
• The collection container should be kept in the refrigerator or a cool place to slow down bacterial growth

Factors Affecting Urinary Oxalate Excretion

Several factors can affect urinary oxalate excretion, including:

• Dietary oxalate intake, which has been shown to have a small but significant impact on urinary oxalate levels 4
• Vitamin C supplements, which can increase urinary oxalate excretion 3
• Body mass index, total fructose intake, and 24-hour urinary potassium, magnesium, and phosphorus levels, which have been positively associated with urinary oxalate 4
• Calcium intake, which has been inversely associated with urinary oxalate 4
• Age, with older individuals tend to have lower urinary oxalate levels 4

Clinical Significance of Urinary Oxalate

Urinary oxalate levels can have significant clinical implications, including:

• Increased risk of kidney stone formation, particularly calcium oxalate stones 5
• Association with chronic kidney disease, with higher urinary oxalate levels increasing the risk of incident CKD 6
• Potential use as a predictive tool for kidney stone recurrence, with changes in urinary oxalate levels potentially indicating an increased risk of recurrence 2