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 laboratory 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 to monitor various analytes in blood and urine and are often used in outpatient clinics.

Table 4.2 lists urine constituents that can be commonly measured away from the laboratory. Many are conveniently measured, semi-quantitatively, using test strips which are dipped briefly into a fresh urine sample. Any excess urine is removed, and the result assessed after a specified time by comparing a colour change with a code on the side of the test strip container. The information obtained from such tests is of variable value to the tester, whether patient or clinician.

The tests commonly performed away from the laboratory can be categorized as follows:

Methodology

It is a feature of many sideroom tests that their simplicity disguises the use of sophisticated methodology. One type of home pregnancy test method involves an elegant application of monoclonal antibody technology to detect the human chorionic gonadotrophin (HCG), which is produced by the developing embryo (Fig 4.2). The test is simple to carry out; a few drops of urine are placed in the sample window, and the result is shown within 5 minutes. The addition of the urine solubilizes a monoclonal antibody for HCG, which is covalently bound to tiny blue beads. A second monoclonal antibody specific for another region of the HCG molecule, is firmly attached in a line at the result window. If HCG is present in the sample it is bound by the first antibody, forming a blue bead–antibody–HCG complex. As the urine diffuses through the strip, any HCG present becomes bound at the second antibody site and this concentrates the blue bead complex in a line – a positive result. A third antibody recognizes the constant region of the first antibody and binds the excess, thus providing a control to show that sufficient urine had been added to the test strip, the most likely form of error.

General problems

The obvious advantages in terms of time saving and convenience to both patient and clinician must be balanced by a number of possible problems in the use of these tests. They include:

Analytical problems

Many problems under this heading will have little to do with the assay technology but will be due to operator errors. Tests designed for use outside the laboratory are robust but are by no means foolproof. Most operators will not be trained laboratory technicians but patients, nurses or clinicians. If an assay is to be performed well these individuals must be trained in its use. This may require the reading of a simple set of instructions (e.g. a home pregnancy test) or attending short training sessions (e.g. the ward-based blood gas analyser). The most commonly encountered analytical errors arise because of failure to:

All of these problems can be readily overcome by following instructions carefully. Regular maintenance of the equipment may be necessary, and simple quality control checks should be performed. It should always be possible to arrange simple quality control cross checks with the main biochemistry laboratory.

Interpretive problems

Even when analytically correct results are obtained, there are other problems which must be overcome before the exercise can be considered a success. The general appropriateness of the test must be considered. If an assay is performed in an individual of inappropriate age, sex, or at the wrong time of day, or month, then the result may be clinically meaningless. Similarly, the nature of the sample collected for analysis should be considered when interpreting the result. Where the results seem at odds with the clinical situation, interference from contaminants (e.g. detergents in urine containers) should be considered as should cross reactivity of the assay with more than one analyte (e.g. haemoglobin and myoglobin).

Any biochemical assay takes all these potential problems into account. However, with extra-laboratory testing, correct interpretation of the result is no longer the laboratory’s responsibility but that of the operator.

The future

There is no doubt that in the future, biochemical testing of patients at the point of care will become practical for many of the analytes currently measured in the laboratory. There is, however, likely to be much debate about costs and the clinical usefulness of such non-laboratory-based analyses.