The clinical biochemistry laboratory in a typical general hospital in the UK serves a population of about 400 000 containing approximately 60 general practitioner (GP) groups, depending upon the location in the UK. This population generates approximately 1200 requests from GPs and hospital doctors each weekday for clinical biochemical tests on their patients. Each patient request requires the laboratory to undertake an average of seven specific analyte tests. The result is that a typical general hospital laboratory carries out between 2.5 and 3 million analyte tests each year. The majority of clinical biochemistry laboratories offers the local medical community as many as 200 different clinical biochemical tests that can be divided into eight categories, as shown in Table 1.
Table1. Examples of biochemical analytes used to support clinical diagnosis
Most of the requests for biochemical tests arise on a routine daily basis, but some will arise from emergency medical situations at any time of the day. The large number of daily test samples coupled with the need for a 24-hour 7-days a week service dictates that the laboratory must rely heavily on automated ana lysis to carry out the tests and on information technology to process the data.
To achieve an effective service, a clinical biochemistry laboratory has three main functions:
• to advise the requesting GP or hospital doctor on the appropriate tests for a particular medical condition and on the collection, storage and transport of the patient samples for analysis
• to provide a quality analytical service for the measurement of biological analytes in an appropriate and timely way
• to provide the requesting doctor with a data interpretation and advice service on the outcome of the biochemical tests and possible further tests.
The advice given to the clinician is generally supported by a user handbook, pre pared by senior laboratory personnel, that includes a description of each test offered, instructions on sample collection and storage, normal laboratory working hours and the approximate time it will take the laboratory to undertake each test. This turn around time varies from less than one hour to several weeks, depending upon the speciality of the test. The vast majority of biochemical tests are carried out on serum or plasma derived from a blood sample. Serum is the preferred matrix for biochem ical tests, but the concentrations of most test analytes are almost the same in the two fl uids. Serum is obtained by allowing the blood to clot and removing the clot by centrifugation. To obtain plasma , it is necessary to add an anticoagulant to the blood sample and remove red cells by centrifugation. The two most common anti coagulants are heparin and EDTA, the choice depending on the particular biochemical test required. For example, EDTA complexes calcium ions, therefore calcium in ‘EDTA plasma’ would be undetectable. For the measurement of glucose, fluoride-oxalate is added to the sample; the oxalate as an anticoagulant, the fluoride to inhibit glycolysis during the transport and storage of the sample. Special vacuum collection tubes containing specific anticoagulants or other additives are available for the storage of blood samples. Collection tubes are also available containing clot enhancers to speed the clotting process for serum preparation. Many containers incorporate a gel with a specific density designed to float between cells and serum providing a barrier between the two for up to 4 days. During these 4 days, the cellular component will experience lysis, so any subsequent contamination of the serum will include intracellular com ponents. Some biochemical tests may also require whole blood, urine, cerebrospinal fluid (the fluid surrounding the spinal cord and brain), faeces, sweat, saliva and amniotic fluid. It is essential that the samples are collected in the appropriate container at the correct time (particularly important if the test is for the measurement of hormones such as cortisol that are subject to diurnal release) and labelled with appropriate patient details. Samples submitted to the laboratory for biochemical tests are accompanied by a request form, signed by the requesting clinician, which gives details of the tests required and brief details of the reasons for the request to aid data interpretation and to help identify other appropriate tests.
Laboratory Reception
On receipt in the laboratory, both the sample and the request form are assigned an acquisition number, usually in an optically readable form that includes a bar code. A check is made of the validity of the sample details on both the request form and sample container to ensure that the correct container for the tests required has been used. Samples may be rejected at this stage, if details are not in accordance with the set protocol. Correct samples are then split from the request form and prepared for analysis, typically by centrifugation to prepare serum or plasma. The request form is processed into the computer system, which identifies the patient against the sample acquisition number, and the tests requested by the clinician typed into the database. It is vital at this reception phase that the sample and patient data match and that the correct details are placed in the database. These details must be adequate to uniquely identify the patient, bearing in mind the number of potential patients in the catchment area, and will include name, address, date of birth, NHS number (a unique identifier for each individual in the UK for health purposes), hospital or accident and emergency number and acquisition number.
