The use of the laboratory

Published on 01/03/2015 by admin

Filed under Basic Science

Last modified 22/04/2025

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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 analyses, it is necessary that the laboratory be provided with both the correct specimen 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 interpretation 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 biochemistry 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.

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 questions. Some of these problems arise when a clinician first obtains specimens from the patient.

image Blood sampling technique. Difficulty in obtaining a blood specimen may lead to haemolysis with consequent release of potassium and other red cell constituents.

image 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.

image Insufficient specimen. It may prove to be impossible for the laboratory to measure everything requested on a small volume.

image 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.

image 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 EDTA (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 EDTA is a chelator of calcium and is present as its potassium salt.

image 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.

image 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.

Unnecessary testing

There can be no definite rules about the appropriateness, or otherwise, of laboratory 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.