Let's talk about lab water
Let's talk about lab water
At ELGA, we often get asked what is CLSI Water? It is Clinical Laboratory Reagent Water (CLRW) as defined in CLSI (Clinical and Laboratory Standards Institute) document “Preparation and Testing of Reagent Water in the Clinical Laboratory; Approved Guideline-Fourth Edition”, issued in 2006. This is still a key reference document for water purity in clinical laboratories. Collegiate of American Pathologists (CAP) accreditation is used in the USA and in laboratories in many other countries. The CAP recommendation is that, as a minimum, water in the laboratory should meet Clinical Laboratory Reagent Water (CLRW) as specified by CLSI.
Bacteria <10 CFU/ml
Resistivity >10 MΩ.cm
TOC <500 PPB
Particles 0.2um filtration or better
The CLSI guideline also states that for instrument feed water – “Use of CLRW for this application must be confirmed with the manufacturer of a specific instrument. Water meeting the manufacturer’s specifications must be used.” CLSI clearly anticipate that CLRW may not be pure enough for all analyzer feeds. The onus is on the analyzer company to validate their chemistries and use purified water of a suitable purity to give accurate and reproducible results.
The CLRW specification limits four key types of impurity in pure water:
All will impact on clinical analyzer performance, either by direct interference with the chemistries of tests or indirectly, by introducing errors in the measurements.
The CLRW resistivity specification of >10MΩ.cm restricts the concentrations of ionic impurities to ppb levels or less and, in effect, requires the elimination of carbon dioxide. This is adequate for most clinical work including general chemical, electrolyte, lipid and protein assays, enzymology, enzyme immunoassay, toxicology and therapeutic drug monitoring and, more recent, molecular biological techniques. When trace elements need to be determined, the water resistivity needs to be much higher – at 18.2 MΩ.cm.
The absence of particulates is a general requirement for all types of application and is especially critical with the low liquid volumes used in modern assays. Particles can clog needles and sample handling manifolds. Deposits of particles encourage the formation of bio-film and bacterial growth and can affect the transmissivity and path length of spectroscopic cells. CLRW relies on filtration to remove particulates. However, the 0.2 µm filters specified may not always be adequate.
Similarly, the TOC spec of <500 ppb in CLRW is a reflection of earlier standards and allows scope for the presence of a wide variety of organic compounds such as carboxylic acids and polyaromatics, which could jeopardize assays. Carboxylic acids can interfere with enzymology and enzyme immunoassays by binding to active sites, and complexing with co-factor metals. Other organics can inhibit enzymes and affect fluorescent detection.
Bacterial contamination has serious effects on all aspects of analyser operation. The key is achieving consistently low levels. For example, problems can arise in immunoassay due to fluorescein released from bacteria (e.g. pseudomonas aureuginosa) giving high blanks and out-of-range standards during calibration and false positives with samples.
Specifications or recommendations are also provided for five other types of purified water to meet different needs in clinical laboratory testing but CLRW is the only one specified in detail. Throughout the document CLSI emphasises the need for good practice and rigorous trending of water system parameters for all types of water to ensure that water purity is achieved and maintained. Water must be validated as fit for purpose and water purification system validation is strongly recommended.
To discuss your clinical laboratory water requirements, please contact ELGA or an Approved Partner in your country.