Summary Clinical chemistry principles techniques correlations

milliliters (mL) to 1.0 liter (L), the process is reversed and the decimal point would be moved three places to the left to become 1.0 L. Note that the SI term for mass is kilogram; it is the only basic unit that contains a prefix as part of its name. Generally, the standard prefixes for mass use the term gram rather than kilogram. Example 1: Convert 1.0 L to μL 1.0 L (1 × 100) = ? μL (micro = 10−6); move the decimal place six places to the right and it becomes 1,000,000 μL; reverse the process to determine the expression in L (move the decimal six places to the left of 1,000,000 μL to get 1.0 L). Example 2: Convert 5 mL to μL 5 mL (milli = 10−3, larger) = ? μL (micro = 10−6, smaller); move the decimal by three places to the right and it becomes 5,000 μL. Example 3: Convert 5.3 mL to dL 5.3 mL (milli = 10−3, smaller) = ? dL (deci = 10−1, larger); move the decimal place by two places to the left and it becomes 0.053 dL. Reporting of laboratory results is often expressed in terms of substance concentration (e.g., moles) or the mass of a substance (e.g., mg/dL, g/dL, g/L, mmol/L, and IU) rather than in SI units. These familiar and traditional units can cause confusion during interpretation. Appendix D (on thePoint), Conversion of Traditional Units to SI Units for Common Clinical Chemistry Analytes, lists both reference and SI units together with the conversion factor from traditional to SI units for common analytes. As with other areas of industry, the laboratory and the rest of medicine are moving toward adopting universal standards promoted by the International Organization for Standardization, often referred to as ISO. This group develops standards of practice, definitions, and guidelines that can be adopted by everyone in a given field, providing for more uniform terminology and less confusion. Many national initiatives have recommended common units for laboratory test results, but none have been widely adopted.2 As with any transition, clinical laboratory scientists should be familiar with all the terms currently used in their field. REAGENTS In today's highly automated laboratory, there seems to be little need for reagent preparation by the clinical laboratory scientist. Most instrument manufacturers make the reagents in a ready-to-use form or “kit” where all necessary reagents and respective storage containers are prepackaged as a unit requiring only the addition of water or buffer to the prepackaged components for reconstitution. A heightened awareness of the hazards of certain chemicals and the numerous regulatory agency requirements has caused clinical chemistry laboratories to readily eliminate massive stocks of chemicals and opt instead for the ease of using prepared reagents. Periodically, especially in hospital laboratories involved in research and development, biotechnology applications, specialized analyses, or method validation, the laboratorian may still face preparing various reagents or solutions. Chemicals Analytic chemicals exist in varying grades of purity: analytic reagent (AR); ultrapure, chemically pure (CP); United States Pharmacopeia (USP); National Formulary (NF); and technical or commercial grade.3 A committee of the American Chemical Society (ACS) established specifications for AR grade chemicals, and chemical manufacturers will either meet or exceed these requirements. Labels on reagents state the actual impurities for each chemical lot or list the maximum allowable impurities. The labels should be clearly printed with the percentage of impurities present and either the initials AR or ACS or the term For laboratory use or ACS Standard-Grade Reference Materials. Chemicals of this category are suitable for use in most analytic laboratory procedures. Ultrapure chemicals have been put through additional purification steps for use in specific procedures such as chromatography, atomic absorption, immunoassays, molecular diagnostics, standardization, or other techniques that require extremely pure chemicals. These reagents may carry designations of HPLC (high-performance liquid chromatography) or chromatographic on their labels. Because USP and NF grade chemicals are used to manufacture drugs, the limitations established for this group of chemicals are based only on the criterion of not being injurious to individuals. Chemicals in this group may be pure enough for use in most chemical procedures; however, it should be recognized that the purity standards are not based on the needs of the laboratory and, therefore, may or may not meet all assay requirements. Reagent designations of CP or pure grade indicate that the impurity limitations are not stated and that preparation of these chemicals is not uniform. It is not recommended that clinical laboratories use these chemicals for reagent preparation unless further purification or a reagent blank is included. Technical or commercial grade reagents are used primarily in manufacturing and should never be used in the clinical laboratory. Organic reagents also have varying grades of purity that differ from those used to classify inorganic reagents. These grades include a practical grade with some impurities; CP, which approaches the purity level of reagent grade chemicals; spectroscopic (spectrally pure) and chromatographic grade organic reagents, with purity levels attained by their respective procedures; and reagent grade (ACS), which is certified to contain impurities below certain levels established by the ACS. As in any analytic method, the desired organic reagent purity is dictated by the particular application. Other than the purity aspects of the chemicals, laws related to the Occupational Safety and Health Administration (OSHA)4 require manufacturers to indicate any physical or biologic health hazards and precautions needed for the safe use, storage, and disposal of any chemical. A manufacturer is required to provide technical data sheets for each chemical manufactured on a document called a Safety Data Sheet (SDS). Reference Materials Unlike other areas of chemistry, clinical chemistry is involved in the analysis of biochemical by-products found in biological fluids, such as serum, plasma, or urine, making purification and a known exact composition of the material almost impossible. For this reason, traditionally defined standards used in analytical chemistry do not readily apply in clinical chemistry. A primary standard is a highly purified chemical that can be measured directly to produce a substance of exact known concentration and purity. The ACS has purity tolerances for primary standards, because most biologic constituents are unavailable within these tolerance limitations; the National Institute of Standards and Technology (NIST)-certified standard reference materials (SRMs) are used instead of ACS primary standard materials.5, 6, 7 The NIST developed certified reference materials/SRMs for use in clinical chemistry laboratories. They are assigned a value after careful analysis, using state-of-the-art methods and equipment. The chemical composition of these substances is then certified; however, they may not possess the purity equivalent of a primary standard. Because each substance has been characterized for certain chemical or physical properties, it can be used in place of an ACS primary standard in clinical work and is often used to verify calibration or accuracy/bias assessments. Many manufacturers use an NIST SRM when producing calibrator and standard materials, and in this way, these materials are considered “traceable to NIST” and may meet certain accreditation requirements. There are SRMs for a number of routine analytes, hormones, drugs, and blood gases, with others being added.5 Water Specifications8 Water is the most frequently used reagent in the laboratory. Because tap water is unsuitable for laboratory applications, most procedures, including reagent and standard preparation, use water that has been substantially purified. There are various methods for water purification including distillation, ion exchange, reverse osmosis, ultrafiltration, ultraviolet light, sterilization, and ozone treatment. Laboratory requirements generally call for reagent grade water that, according to the Clinical and Laboratory Standards Institute (CLSI), is classified into one of six categories based on the specifications needed for its use rather than the method of purification or preparation.9,10 These categories include clinical laboratory reagent water (CLRW), special reagent water (SRW), instrument feed water, water supplied by method manufacturer, autoclave and wash water, and commercially bottled purified water. Laboratories need to assess whether the water meets the specifications needed for its application. Most water-monitoring parameters include at least microbiological count, pH, resistivity (measure of resistance in ohms and influenced by the number of ions present), silicate, particulate matter, and organics. Each category has a specific acceptable limit. A long-held convention for categorizing water purity was based on three types, I through III, with type I water having the most stringent requirements and generally suitable for routine laboratory use. Prefiltration can remove particulate matter from municipal water supplies before any additional treatments. Filtration cartridges are composed of glass; cotton; activated charcoal, which removes organic materials and chlorine; and submicron filters (≤0.2 mm), which remove any substances larger than the filter's pores, including bacteria. The use of these filters depends on the quality of the municipal water and the other purification methods used. For example, hard water (containing calcium, iron, and other dissolved elements) may require prefiltration with a glass or cotton filter rather than activated charcoal or submicron filters, which quickly become clogged and are expensive to use. The submicron filter may be better suited after distillation, deionization, or reverse osmosis treatment. Distilled water has been purified to remove almost all organic materials, using a technique of distillation much like that found in organic chemistry laboratory distillation experiments in which water is boiled and vaporized. Many impurities do not rise in the water vapor and will remain in the boiling apparatus so that the water collected after condensation has less contamination. Water may be distilled more than once, with each distillation cycle removing additional impurities. Ultrafiltration and nanofiltration, like distillation, are excellent in removing particulate matter, microorganisms, and any pyrogens or endotoxins. Deionized water has some or all ions removed, although organic material may still be present, so it is neither pure nor sterile. Generally, deionized water is purified from previously treated water, such as prefiltered or distilled water. Deionized water is produced using either an anion or a cation exchange resin, followed by replacement of the removed ions with hydroxyl or hydrogen ions. The ions that are anticipated to be removed from the water will dictate the type of ion exchange resin to be used. One column cannot service all ions present in water. A combination of several resins will produce different grades of deionized water. A two-bed system uses an anion resin followed by a cation resin. The different resins may be in separate columns or in the same column. This process is excellent in removing dissolved ionized solids and dissolved gases