Urine acidification is often meant to preserve catecholamines before they are assayed. Catecholamines are organic compounds synthesized from tyrosine and play a significant role as a hormone or neurotransmitter. The most common are adrenaline (epinephrine), norepinephrine (norepinephrine) and dopamine.
Urine acidification is an urinary chemistry process. There are different concentrations of 10N and 6N acids. This concentration must be controlled to reach the expected PH range for the method.
This generates some pitfalls for the latter accreditation, and around the calculation of the volume of acid to be used according to its concentration and knowing the urine sample PH.
Acidifying before analysis: it is risky!
Adding acid as a first line is risky for the patient. The acid projection risk is obvious during collection, and even higher risks may arise from the collection protocol misunderstanding by the patient – imagine for a moment that the latter drinks the acid … It would be necessary to add a pictogram on the container (the regulation is the transposition of the Globally Harmonized System of Classification and Labelling of Chemical GHS).
In addition, the acid volume would depend on the urine total volume that the patient has emitted and the PH.
These are difficult parameters to control, so it represents a risk for the dosage because if there is too much acid at the beginning, it will be necessary to buffer in excess (with a base), but some methods are sensitive to a basic excess.
Why to acidify 24H urine samples?
The central question is to understand whether acidification makes it possible to better preserve urine. There are two publications that seem to demonstrate that it is not essential to acidify urine and that temperature also intervenes in the preservation process.
CLIN. CHEM. 39/12, 2502-2508 (1993)
Optimal Collection and Storage Conditions for Catecholamine Measurements in Human Plasma and Urine
Frans Boosmsms, Gootzen, Loes Van Eijk, Arie J. Man in ‘t Veld, and Maarten A.D.H.Shalekamp
Improvements in methodologies for measuring catecholamines concentrations (CA) have led to increasing use of these compounds as markers in the patients screening and in long-term clinical trials. Because of the associated logistical problems, we have investigated the unresolved question of optimal conditions for sample preparation and for plasma and urine samples storage.
Results show that blood should be centrifuged within 1 hour after collection ; the refrigerated centrifuge use is not necessary. Once plasma is prepared, CA are stable for 1 day at 20°C, 2 days at 4°C, 1 month at -20°C (or 6 months with added gluthacione), an t o 1 year at -70°C.
CA are stable at 4°C for 1 month in unpreserved urine and for 4 months in urine preserved with EDTA and sodium metabisulfite. In acidified urine, CA were nearly unchanged after 1 year at 4 and -20°C.
However, it is necessary to cross-check the recommendations, the bibliographies with the reagents technical sheets dedicated to the dosages. (Chromatography, HPLC etc.)
Some reagents require sample acidification within a pH range defined by the method. This acidification is an analytical factor unrelated to conservation. Acidification can be carried out in the laboratory (on sample) to limit the risks.
In addition, the WHO and W.GUDER (German United Society for Clinical Chemistry and Laboratory Medicine) indicate 4 days stability at room temperature without acidification and that it is not strictly necessary to acidify with chloric acid (dangerous) but also with EDTA.
Catecholamines appear very unstable by nature and can degrade in the patient’s bladder. The collection phase – the one that takes place far from the mastery of good practices of the biologist – requires that the patient urinate every 3 hours? This protocol is rarely respected and is a source of many pre-analytical errors.
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