Simplified Urine Collection for Kidney Stone Test May Be Possible

New Cleveland Clinic research shows that it may be feasible to shorten the urine-collection regimen that kidney stone-forming patients follow to evaluate recurrence risk.

Urine specimens collected during a 12-hour nighttime period were comparable to 24-hour collection samples in terms of their sensitivity for detection of elevated supersaturation levels of three common stone-forming metabolites.

“We found strong correlation between the nighttime 12-hour and 24-hour collections for all three supersaturations,” says Bryan Hinck, MD, a urology resident at Cleveland Clinic’s Glickman Urological & Kidney Institute and the study’s lead author.

Reducing the sampling period’s duration by half and allowing nighttime, at-home collection could reduce inconvenience for patients, improve compliance and possibly reduce costs, Dr. Hinck and his colleagues concluded. Their findings were presented at the American Urological Association’s 2016 annual meeting.

Reducing patient burden

The collection of urine samples during a 24-hour period is considered standard practice for the comprehensive metabolic analysis of recurrent nephrolithiasis patients.

The 24-hour collection interval is meant to ensure that the sample represents a patient’s usual hydration and diet, thus providing an accurate indication of typical urine values and metabolites, supersaturation and recurrence risk.

Current American Urological Association guidelines recommend one or two 24-hour urine collections obtained on a random diet and analyzed at minimum for total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium and creatinine.

While the 24-hour collection interval may be optimal for analytical purposes, it is burdensome for patients and could hinder compliance and accuracy. Collecting urine while at work is intrusive; a recent study found that patients who performed urine collection on a Sunday were much more likely to supply a sample amount that was adequate for diagnostic purposes. The same study found that a majority of patients submit inadequate 24-hour urine collections.

“The 24-hour collection is a great test and provides a wealth of information when done properly,” says Dr. Hinck. “Anecdotally, patients say the daytime collection, especially during the weekday, is the hardest to complete. They’re often at work, and they have to figure out how to keep a jug of urine in the work refrigerator. It’s difficult.”

The researchers sought to determine if urine samples collected during a 12-hour overnight sampling period were as sensitive to elevated supersaturation levels of calcium oxalate (CaOx), calcium phosphate (CaP) and uric acid (UA) as traditional 24-hour specimens are.

The investigators collected urine from study participants with a history of stone formation. The samples were collected during two 24-hour periods, with each specimen divided into two 12-hour collections: a daytime one beginning after the first morning void and ending 12 hours later with a terminal void, and a subsequent nighttime/overnight collection that finished after the first morning void the following day at the 24 hour point.

In addition to overnight sampling’s convenience, the researchers theorized that the specimens might be more representative of patients’ stone recurrence risk than daytime samples are. “It’s long been speculated that the first morning void is the most concentrated since patients aren’t hydrating overnight,” Dr. Hinck says. “Our hypothesis therefore was that the overnight samples might be more sensitive and better pick up some of the parameters for increased stone risk.”

Comparing supersaturation levels and sensitivity

49 samples from 36 patients met criteria for inclusion in the study. Overall, there was strong correlation between the night time chemistries and the 24-hour chemistries.

There was strong correlation between the night time SS values and 24 hour SS values as seen in table 1.

Table 1

Five collections had elevated 24 hr SSCaOx levels (> 10.0); in all cases, and 2 additional cases, the night time SSCaOx was also >10.0. 24 collections had elevated 24 hr SSCaP levels (>1.0); in 20 cases the night time SSCaP was also >1.0 and an additional collection was identified. Nine collections had elevated 24 hr SSUA levels (> 1.0); in 8 of these collections the night time SSUA was also >1.0, however an additional collection was identified as >1.0 on the night time collection. Differences between night and 24-hour collection varied based on individual chemistry value ranging from 2.0%-18.4% of patients having 30% difference between the two collections.

More research is needed

While the results are promising, additional investigation is needed to determine whether a 12-hour nighttime regimen is reliable enough to replace the 24-hour test.

“With a 12-hour collection there will be inherent variability,” Dr. Hinck says. “Clearly there’s not enough literature yet as to what that variability is and what amount is tolerable. It warrants further study. Hopefully our results will spur more research.”