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 analyzer operation. The key is achieving consistently low levels. For example, problems can arise in immunoassay due to flurescein-based dye 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 emphasizes 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. Ultrapure Water must be validated as fit for purpose and water purification system validation is strongly recommended.
Dr Paul Whitehead
After a BA in Chemistry at Oxford University, Paul focused his career on industrial applications of chemistry. He was awarded a PhD at Imperial College, London for developing a microwave-induced-plasma detector for gas chromatography. He spent the first half of his career managing the analytical support team at the Johnson Matthey Research/Technology Centre,specialising in the determination of precious metals and characterising applications such as car-exhaust catalysts and fuel cells. Subsequently, as Laboratory Manager in R&D for ELGA LabWater, he has been involved in introducing and developing the latest water purification technologies. He now acts as a consultant for ELGA.