Introduction
Biochemical tests are crucial to many areas of modern medicine. Most biochemical tests are carried out on blood using plasma or serum, but urine, cerebrospinal fluid (CSF), faeces, kidney stones, pleural fluid, etc. are sometimes required. Plasma is obtained by taking whole blood into an anti-coagulant and represents the aqueous supernatant obtained when all the cellular elements have been separated by centrifugation. Serum is the corresponing aqueous phase when blood is allowed to clot. For many (but not all) biochemical tests on blood, it makes little difference whether plasma or serum is used and the terms are often used interchangeably.
There are many hundreds of tests available in clinical biochemistry that include many specialist tests. However, a core of common tests makes up the majority of test requesting in clinical biochemistry. These core tests will be offered by almost all clinical biochemistry laboratories and will be available 24h daily for more urgent situations. It is also sometimes appropriate to bring tests together in profiles, especially where a group of tests can provide better understanding of a problem than a single test (e.g. the liver function test profile). Many of the other more specialist tests are restricted to larger laboratories or, in some cases, to a very small number of centres offering a regional or national service.
In dealing with the large number of routine test requests, the modern clinical biochemistry laboratory depends heavily on automated instrumentation. This is most often linked to a laboratory computing system which assigns test requests to electronic patient files, maintains a cumulative patient record and regulates the printing of reports. Increasingly, test requests can be electronically booked at the ward, clinic or even general practitioner (GP) surgery via a terminal linked to the main laboratory computer. Equally, the test results can be displayed on computer screens at distant locations, even negating the need for issuing printed reports.
In this first chapter, we set out some of the principles of requesting tests and of the interpretation of results. The effects of analytical errors and of physiological factors, as well as of disease, on test results are stressed. Biochemical testing in differential diagnosis and in screening is discussed.
Collection of specimens
Test requests require unambiguous identification of the patient (patient’s name, sex, date of birth and, increasingly, a unique patient identification number), together with the location, the name of the requesting doctor and the date and time of sampling. Each test request must specify which analyses are requested and provide details of the nature of the specimen itself and relevant clinical diagnostic information. Traditionally, this information is provided through the request form with appropriate parallel labelling of the specimen itself. Increasingly, this information is provided electronically so that only the sample itself need be sent to the laboratory with its own unique identifier (e.g. a bar code which links it to the electronic request).
Because of the large number of samples which are processed by most clinical biochemistry laboratories, every step needs to be taken to avoid errors. Regrettably, errors do rarely occur and can be dividied according to the error source:
- Pre-analytical. For example, assigning a specimen to the wrong patient at the ward end or taking a sample at the wrong time (e.g. digoxin level is requested on a sample shortly after digoxin has been administered (pp. 279)) or mislabelling of an aliqout of serum taken at specimen reception. Most errors fall into this category (see Table 1.1).
- Analytical. For example, a small sample volume may lead to a pipetting error where insufficient sample is used for the assay. Again, developments in automated sample detection and pipetting mean these problems are very unusual.
- Post-analytical. These are increasingly rare because of electronic download of results from the analyser but might include transcription errors when entering results into the lab computer manually.
On the scale of the requesting of biochemical tests, errors are fortunately rare. However, occasional blunders do arise and, if very unexpected results are obtained, it is incumbent on the requesting doctor to contact the laboratory immediately to look into the possibility that a blunder may have occurred.
Table 1.1 Some more common causes of pre-analytical errors arising from use of the laboratory
| Error | Consequence |
| Crossover of addressograph labels between patients | This can lead to two patients each with the other’s set of results. Where the patient is assigned a completely wrong set of results, it is important to investigate the problem in case there is a second patient with a corresponding wrong set of results. |
| Timing error | There are many examples where timing is important but not considered. Sending in a blood sample too early after the administration of a drug can lead to misleadingly high values in therapeutic monitoring. Interpretation of some tests (e.g. cortisol) is critically dependent on the time of day when the blood was sampled. |
| Sample collection tube error | For some tests the nature of the collection tube is critical, which is why the Biochemistry Laboratory specifies this detail. For example, using a plasma tube with lithium-heparin as the anti-coagulant invalidates this sample tube for measurement of a therapeutic lithium level! Electrophoresis requires a serum sample; otherwise, the fibrinogen interferes with the detection of any monoclonal bands. Topping up a biochemistry tube with a haematology (potassium ethylenediamine tetraacetic acid (EDTA) sample) will lead to high potassium and low calcium values in the biochemistry sample. |
| Sample taken from close to the site of an intravenous (IV) infusion | The blood sample will be diluted so that all the tests will be correspondingly low with the exception of those tests which might be affected by the composition of the infusion fluid itself. For example, using normal saline as the infusing fluid would lead to a lowering of all test results, but with sodium and chloride results which are likely to be raised. |
The use of clinical biochemistry tests
Biochemical tests are most often discretionary, meaning that the test is requested for defined diagnostic purposes, as distinct from screening, where a disease is sought without there being any specific indication of its presence in the individual. The justification for discretionary testing is well summarised by Asher (1954):
- To assist in diagnosis. For example, the diagnosis of diabetes mellitus is crucially dependent on the measurement and interpretation of plasma [glucose]. Biochemical tests may also aid the differential diagnosis or indicate the severity of a disease (see also Table 1.2).
- In disease monitoring. A good example is the use of arterial blood gases to follow the progress of someone admitted with a severe pneumonia or creatinine in an individual with chronic renal failure (see also Table 1.2).
- In prognosis or disease risk assessment. Serum cholesterol (pp. 192) or high-sensitive C-reactive protein (hsCRP) (pp. 192) are used in the assessment of cardiovascular risk, for example.
- In screening for disease. An example here would be measurement of thyroid-stimulating hormone (TSH) to screen for neonatal hypothyroidism.
- Miscellaneous, for example for forensic purposes or ethically approved research.
- In well-population screening a spectrum of tests is carried out on individuals from an apparently healthy population in an attempt to detect pre-symptomatic or early disease. It is easy to miss significant abnormalities in the ‘flood’ of data coming from the laboratory, even when the abnormalities are ‘flagged’ in some way. Most of the abnormalities detected will be of little or no significance, yet may need additional time-consuming and often expensive tests to clarify their importance (or lack of it). For these and other reasons, the value of well-population screening has been called into question and certainly should only be initiated under certain specific circumstances which are listed in Table 1.3.
- In case-finding screening programmes appropriate tests are carried out on a population sample known to be at high risk of a particular disease. These are inherently more selective and yield a higher proportion of useful results (Table 1.4).
Table 1.2 Test selection for the purposes of discretionary testing
| Category | Example |
| To confirm a diagnosis | Serum [free T4] and [thyroid-stimulating hormone, (TSH)] in suspected hyperthyroidism |
| To aid differential diagnosis | To distinguish between different forms of jaundice |
| To refine a diagnosis | Use of adrenocorticotrophic hormone (ACTH) to localise Cushing′s syndrome |
| To assess the severity of disease | Serum [creatinine] or [urea] in renal disease |
| To monitor progress | Plasma [glucose] and serum [K+] to follow treatment of patients with diabetic ketoacidosis (DKA) |
| To detect complications or side effects | Alanine aminotransferase (ALT) measurements in patients treated with hepatotoxic drugs |
| To monitor therapy | Serum drug concentrations in patients treated with anti-epileptic drugs |
Table 1.3 Requirements for well-population screening
| The disease is common or life-threatening |
| The tests are sensitive and specific |
| The tests are readily applied and acceptable to the population to be screened |
| Clinical, laboratory and other facilities are available for follow-up |
| Economics of screening have been clarified and the implications accepted |
Table 1.4 Examples of tests used in case-finding programmes
| Programmes to detect diseases in | Chemical investigations |
| Neonates | |
| PKU | Serum [phenylalanine] |
| Hypothyroidism | Serum [TSH] and/or [thyroxine] |
| Adolescents and young | |
| adults | |
| Substance abuse | Drug screen |
| Pregnancy | |
| Diabetes mellitus in the mother | Plasma and urine [glucose] |
| Open neural tube defect (NTD) in the foetus | Maternal serum [α-fetoprotein] |
| Industry | |
| Industrial exposure to lead | Blood [lead] |
| Industrial exposure to pesticides | Serum cholinesterase activity |
| Elderly | |
| Malnutrition | Serum vitamin D levels |
| Thyroid dysfunction | Serum [TSH] and/or [thyroxine] |
Point of care testing (POCT) (Table 1.5)
There are occasions when the urgency of the clinical situation requires that blood testing on patient samples is performed near the patient (point of care testing). Furthermore in the UK the government, in outlining the future of the National Health Service, has indicated a desire to move laboratory testing from the hospital laboratory into the community setting. High street pharmacies have taken up these opportunities and can, for example, provide cholesterol and glucose testing while you wait. In addition, there is an increasing number of urine test sticks that are sold for home use (e.g. pregnancy and ovulation testing by measuring human chorionic gonadotrophin (hCG) and luteinising hormone (LH), respectively).
POCT eliminates the need to send the specimen to the laboratory, and will usually allow a more rapid turnaround time. POCT is particularly suitable for use in intensive care units (ICUs), high-dependency units (HDUs), Accident and Emergency (A&E) departments and specialist clinics. Small dedicated analysers are often introduced into these centres.
Since decisions to initiate treatment are often made on the basis of POCT it is vital that confidence can be placed in the results obtained by such methods, such that the risk to the patient is minimised. It is thus essential that POCT is carried out by staff who are suitably trained and that the reliability of the tests is monitored on a regular basis using appropriate quality control measures.
If POCT is to be introduced into a ward or outpatient department it is essential that:
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