Two practical examples show how correct sample preparation helps prevent inaccurate laboratory results

When collecting urine samples, particularly 24-hour urine collections, it is important to follow the requirements regarding the addition of stabilising agents such as hydrochloric acid. This process is often perceived as cumbersome by those involved. But why does it matter so much whether a urine sample has been stabilised or not? Two examples illustrate how correct sample preparation can directly affect the reliability of laboratory results.

Catecholamines in urine

When collecting a 24-hour urine sample for the measurement of the catecholamines adrenaline, noradrenaline and dopamine, the urine must be acidified from the very first collected portion. Hydrochloric acid must therefore already be present in the collection container before collection begins.

There is an important reason for this. Adrenaline, noradrenaline and dopamine are not stable in urine. Among other processes, oxidation causes them to break down further into metanephrine, normetanephrine and 3-methoxytyramine.

These breakdown products are not included when catecholamines are quantified. As a result, falsely low values may be measured because some of the catecholamines have already degraded during the collection period.

Adding hydrochloric acid prevents this degradation. Acidifying the urine afterwards is not sufficient, as the breakdown may already have occurred.

Oxalate in urine

A 24-hour urine sample collected for oxalate measurement must also be acidified. Unlike catecholamine testing, however, the urine does not need to remain acidified throughout the entire collection period. It is sufficient to acidify the collected urine before sending it to the laboratory.

It is essential that the entire urine collection is acidified with hydrochloric acid and then thoroughly mixed. Only acidifying the aliquot must be avoided.

If only an aliquot is sent to the laboratory, the entire urine collection must first be acidified and mixed thoroughly. The aliquot should only be taken afterwards.

The reason lies in the chemical properties of calcium oxalate. Kidney stones are often composed of calcium oxalate, which is only slightly soluble in water. At 20 °C, the solubility of calcium oxalate in water is only approximately 0.0061 g/l.

As a result, calcium oxalate can crystallise in urine and accumulate like sand at the bottom of the collection container.

If an aliquot is then taken with a pipette from the upper part of the container, it may contain substantially less oxalate than is actually present in the urine as a whole. Conversely, the concentration at the bottom of the container may be much higher. This can lead to both falsely low and falsely high results.

When hydrochloric acid is added, precipitated calcium oxalate is converted into calcium chloride and oxalic acid. At approximately 100 g/l at 20 °C, oxalic acid is many times more soluble in water than calcium oxalate.

This ensures that the oxalate is evenly distributed throughout the urine sample and allows the laboratory to obtain reliable analytical results.

Did you know?

This is also one of the reasons why urinary sediment must be examined using non-acidified urine. In acidified urine, calcium oxalate crystals dissolve and would therefore no longer be visible under the microscope.