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.
Blood specimens
If blood is collected into a plain tube and allowed to clot, after centrifugation a serum specimen is obtained (Fig 2.1). For many biochemical analyses this will be the specimen recommended. In other cases, especially when the analyte in question is unstable and speed is necessary to obtain a specimen that can be frozen quickly, the blood is collected into a tube containing an anticoagulant such as heparin. When centrifuged, the supernatant is called plasma, which is almost identical to the cell-free fraction of blood but contains the anticoagulant as well.
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| Fig 2.1 Blood specimen tubes for specific biochemical tests. The colour-coded tubes are the vacutainers in use in the authors’ hospital and laboratory. |
Urine specimens
Urine specimen containers may include a preservative to inhibit bacterial growth, or acid to stabilize certain metabolites. They need to be large enough to hold a full 24-hour collection. Random urine samples are collected into small ‘univer- sal’ containers.
Other specimen types
For some tests, specific body fluids or tissue may be required. There will be specific protocols for the handling and transport of these samples to the labora- tory. Consult the local lab for advice.
Dangerous specimens
All specimens from patients with dan- gerous infections should be labelled with a yellow ‘dangerous specimen’ sticker. A similar label should be attached to the request form. Of most concern to the laboratory staff are hepatitis B and HIV.
Sampling errors
There are a number of potential errors that may contribute to the success or failure of the laboratory in providing the correct answers to the clinician’s ques- tions. Some of these problems arise when a clinician first obtains specimens from the patient.
- Blood sampling technique. Difficulty in obtaining a blood specimen may lead to haemolysis with consequent release of potassium and other red cell constituents.
- Prolonged stasis during venepuncture. Plasma water diffuses into the interstitial space and the serum or plasma sample obtained will be concentrated. Proteins and protein- bound components of plasma, such as calcium or thyroxine, will be falsely elevated.
- Insufficient specimen. It may prove to be impossible for the laboratory to measure everything requested on a small volume.
- Errors in timing. The biggest source of error in the measurement of any analyte in a 24-hour urine specimen is in the collection of an accurately timed volume of urine.
- Incorrect specimen container. For many analyses the blood must be collected into a container with anticoagulant and/or preservative. For example, samples for glucose should be collected into a special
container containing fluoride, which inhibits glycolysis; otherwise the time taken to deliver the sample to the laboratory can affect the result. If a sample is collected into the wrong container, it should never be decanted into another type of tube. For example, blood that has been exposed, even briefly, to E DTA (an anticoagulant used in sample containers for lipids) will have a markedly reduced calcium concentration, approaching zero, along with an artefactually high potassium concentration. This is because E DTA is a chelator of calcium and is present as its potassium salt.
- Inappropriate sampling site. Blood samples should not be taken ‘downstream’ from an intravenous drip. It is not unheard of for the laboratory to receive a blood glucose request on a specimen taken from the same arm into which 5% glucose is being infused. Usually the results are biochemically incredible but it is just possible that they may be acted upon with disastrous consequences for the patient.
- Incorrect specimen storage. A blood sample stored overnight before being sent to the laboratory will show falsely high potassium, phosphate and red cell enzymes, such as lactate dehydrogenase, because of leakage into the extracellular fluid from the cells.
Timing
Many biochemical tests are repeated at intervals. How often depends on how quickly significant changes are liable to occur, and there is little point in request- ing repeat tests if a numerical change will not have an influence on treatment. The main reason for asking for an analy- sis to be performed on an urgent basis is that immediate treatment depends on the result.
Analysing the specimen
Once the form and specimen arrive at the laboratory reception, they are matched with a unique identifying number or bar-code. The average lab receives many thousands of requests and samples each day and it is impor- tant that all are clearly identified and never mixed up. Samples proceed through the laboratory as shown in Figure 2.2. All analytical procedures are quality controlled and the laboratory strives for reliability.
Once the results are available they are collated and a report is issued. Cumula- tive reports allow the clinician to see at a glance how the most recent result(s) compare with those tests performed pre- viously, providing an aid to the monitoring of treatment (see p. 12)
Unnecessary testing
There can be no definite rules about the appropriateness, or otherwise, of labora- tory testing because of the huge variety of clinical circumstances that may arise.
Clinicians should always bear in mind that in requesting a biochemical test they should be asking a question of the laboratory. If not, both the clinician and the laboratory may be performing unnecessary work, with little benefit to the patient.
Clinical note
Clinical biochemistry is but one branch of laboratory medicine. Specimens may be required for haematology, microbiology, virology, immunology and histopathology, and all require similar attention to detail in filling out request forms and obtaining the appropriate samples for analysis.
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