Can you trust measures of urine potassium?

This is another post for my Nutrition Research Method series. These posts review the methods used in research to estimate what people are actually eating.  One of the projects I am currently working on, is exploring how to measure potassium in adults with CKD.

So, today’s post is a summary of this article published in 2020 in Nutrients. 

Refresher

Ok, let’s start with a quick refresher. If we want to know how much potassium someone is eating, we have measure it. In all cases, these measurements are estimates. There is no real way to measure actual potassium intake in free-living humans.

The tools we use to measure fall into two broad categories. Either, we ask people what they are eating or we measure the amount of potassium in urine.

To read more about different diet recall methods – check out this post.

A closer look at urine measures of potassium

A 24 hour urine potassium measurement is considered the gold standard, as urine collections are not subject to participant recall errors.  Urine collections don’t rely on accurate information in nutrient databases.  It is universally accepted that 70% of ingested potassium will be excreted in the urine.  However, several studies have reported that measured intakes and urine samples are not well correlated. 

Why might urine not accurately reflect intake?

1. Sample Completeness: One of the potential errors in 24 hour urine collections is the need for the sample to be complete.  If someone doesn’t collect every urine, then potassium excretion could be underestimated.  If someone collects too much, potassium excretion could be over-estimated. The best method to ensure a complete sample is PABA, but this is expensive. Other methods, such as creatinine, are less reliable.
2. Study Design: Potassium balance studies are conducted with a stable potassium intake over several days.  This may not be reflective of free-living humans who likely have variability in their daily potassium intake. 
3. Nutrient Interactions: Another concern is how other nutrient intake impacts potassium excretion. For example high sodium intake may change how much potassium is excreted in the urine.
4. Timing: Finally, we need to consider when the urine measurement was taken and when the outcome of interest was observed.  If someone wants to understand how urine potassium excretion is related to serum levels, dialysis start or a heart attack, then getting the ideal timing of the urine collection can be challenging.  The authors highlight that many studies collect only 1 urine sample and then look at outcomes several months or years later. The authors question whether this is a fair analysis to make.

What are alternatives to a 24 hour urine collection?

Spot urine collections are considered alternatives to 24 hour urine collections. The advantages of the spot urine is that you don’t have to worry about a complete 24 hour sample. However, they require the use of a formula to translate the spot urine number into a 24 hour urine number.

Various formulas have been used with varying degrees of success.  Though it is important to note, all formulas are designed to match the spot urine to a 24 hour urine. Which isn’t necessarily the same as measured dietary intake.

Specific comments about the CKD population

The authors of this article made special comments about the CKD population, specifically noting that changes in the disease could make estimation of potassium intake through urine more error prone.  There are concerns about changes in circadian rhythm patterns that may make spot urines more likely to give inaccurate results. 

In my next article – I am going to take a look at specific studies in CKD that have measured dietary intake and urine potassium.

Take Aways

Urine potassium measures have potential sources of error, whether it is with collection errors or mathematical errors.

When looking at studies that have used urine to estimate potassium, consider asking yourself these questions:

1. How many urine samples did they take? More improves the reliability of the results
2. If they used a 24 hour urine collection, how did the assess for sample completeness? Was it assessed with PABA? If yes – great! If not, the results are less reliable.
3. If a spot urine was used, what formula did they use? Is that formula giving an accurate result for your population?
4. When was potassium measured and when was the outcome observed? For example, if the urine was measured in December how relevant is it to a serum value collected in May?