Fluid and electrolyte balance: Concepts and vocabulary

Fluid and electrolyte balance is central to the management of any patient who is seriously ill. Measurement of serum sodium, potassium, urea and creatinine, frequently with bicarbonate, is the most commonly requested biochemical profile and yields a great deal of information about a patient’s fluid and electrolyte status and renal function. A typical report is shown in Figure 6.1

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Reference intervalsme

Below, in Tables 5.1 and 5.2, is a list of reference intervals for a selection of tests that are performed in clinical biochem- istry laboratories. Where available, refer- ence intervals have been adopted from those suggested by Pathology Harmony, which is a U K-based project aiming to harmonize reference intervals for common analytes across the U K. In the absence of this approach, individual laboratories should use reference inter- vals that are based on values obtained from subjects appropriately selected from local populations, but this is not always feasible. For some analytes, e.g. glucose and cholesterol, conversion factors are supplied to allow different units to be compared. The list is not intended to be comprehensive; it is merely provided for guidance in answering the cases and examples in this book. Please note that age- and/or sex-specific reference intervals are avail- able for a range of analytes including alkaline phosphatase, creatinine, and urate. The sex-specific ranges for urate are shown in Table 5.1. Glucose, insulin and triglyceride all rise postprandially and should, where possible, be meas- ured in the fasting state.

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Point of care testing

The methods for measuring some bio- logical compounds in blood and urine have become so robust and simple to use that measurements can be made away from the laboratory – by the patient’s bedside, in the ward sideroom, at the GP’s surgery, at the Pharmacy or even in the home. Convenience and the desire to know results quickly, as well as expectation of commercial profit by the manufacturers of the tests, have been the major stimuli for these developments. Experience has shown that motivated individuals, e.g. diabetic patients, fre- quently perform the tests as well as highly qualified professionals.

The immediate availability of results at the point of care can enable the appropriate treatment to be instituted quickly and patients’ fears can be allayed. However, it is important to ensure that the limitations of any test and the sig- nificance of the results are appreciated by the tester to avoid inappropriate intervention or unnecessary anxiety.

Outside the laboratory

Table 4.1 shows what can be commonly measured in a blood sample outside the normal laboratory setting. The most common blood test outside the labora- tory is the determination of glucose concentration, in a finger stab sample, at home or in the clinic. Diabetic patients who need to monitor their blood glucose on a regular basis can do so at home or at work using one of many commercially available pocket-sized instruments.

Figure 4.1 shows a portable bench analyser. These analysers may be used

Fig 4.1 A portable bench analyser.

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The interpretation of results

It can take considerable effort, and expense, to produce what may seem to be just numbers on pieces of paper or on a computer screen. Understanding what these numbers mean is of crucial importance if the correct diagnosis is to be made, or if the patient’s treatment is to be changed.

How biochemical results are expressed

Most biochemical analyses are quantita- tive, although simple qualitative or semi- quantitative tests, such as those for the presence of glucose in urine, are com- monly encountered methods used for point of care testing. Many tests measure the amount of the analyte in a small volume of blood, plasma, serum, urine or some other fluid or tissue. Results are reported as concentrations, usually in terms of the number of moles in one litre (mol/ L) (Table 3.1).

The concept of concentration is illus- trated in Figure 3.1. The concentration of any analyte in a body compartment is a ratio: the amount of the substance

Fig 3.1 Understanding concentrations. Concentration is always dependent on two factors: the amount of solute and the amount of solvent. The concentration of the sugar solution in the beaker can be increased from 1 spoon/ beaker (a) to 2 spoons/beaker by either decreasing the volume of solvent (b) or increasing the amount of solute (c)

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The use of the laboratory

Every biochemistry analysis should attempt to answer a question that the clinician has posed about the patient. Obtaining the correct answers can often seem to be fraught with difficulty. 

Specimen collection 

In order to carry out biochemical analy- ses, it is necessary that the laboratory be provided with both the correct speci- men for the requested test, and also information that will ensure that the right test is carried out and the result returned to the requesting clinician with the minimum of delay. As much detail as possible should be included on the request form to help both laboratory staff and the clinician in the interpreta- tion of results. This information can be very valuable when assessing a patient’s progress over a period, or reassessing a diagnosis. Patient identification must be correct, and the request form should include some indication of the suspected pathology. The requested analyses should be clearly indicated. Request forms differ in design. Clinical biochem- istry forms in Europe are conventionally coloured green.

A variety of specimens are used in biochemical analysis and these are shown in Table 2.1.

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The clinical biochemistry laboratory

Clinical biochemistry, chemical pathology and clinical chemistry are all names for the subject of this book, that branch of laboratory medicine in which chemical and biochemical methods are applied to the study of disease (Fig 1.1). While in theory this embraces all non- morphological studies, in practice it is usually, though not exclusively, con- fined to studies on blood and urine because of the relative ease in obtaining such specimens. Analyses are made on other body fluids, however, such as gastric aspirate and cerebrospinal fluid. Clinical biochemical tests comprise over one-third of all hospital laboratory investigations.

Fig 1.1 The place of clinical biochemistry  in medicine

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SAFETY IN THE LABORATORY

Safety First

The concern for laboratory safety can never be overemphasized. Most students who are involved in biochemistry laboratory activities have progressed through several years of college lab work without even a minor accident. This record is, indeed, something to be proud of; however, it should not lead to overconfidence.

You must always be aware that chemicals used in the laboratory are potentially toxic, irritating, and flammable. However, such chemicals are a hazard only when they are mishandled or improperly disposed of. It is my experience as a lab instructor for 30 years that accidents happen least often to those who come to each lab session mentally prepared and with a complete understanding of the experimental procedures to be followed. Because dangerous situations can develop unexpectedly, though, you must be familiar with general safety practices, facilities, and emergency actions. When we work in the lab, we must also have a special concern for the safety of lab mates. Carelessness on the part of one person can often cause injury to others.

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