What does the study of biochemistry in a lab entail?
Within a biochemistry lab, both blood and urine samples from patients come in every day from different sections of a hospital and these samples are screened for the different types of minerals (referred to as analytes) in the blood. The analytes are measured quantitatively i.e. the amount of analyte that is present within the blood and then compared to known reference values and a diagnosis is given based on these results. The samples can be part of a routine screen, or, as the picture alongside demonstrates, they may come from the Emergency (or A&E) department of a hospital. The latter samples tend to be labelled as urgent because the patients are often in life-threatening situations.
What are the types of things typically screened for in a biochemistry lab?
The table below demonstrates the different types of analytes that are commonly screened for. They include sodium, potassium, bicarbonate, glucose, calcium and magnesium. All the values are measured in millimoles per litre, written as mmolL-1.
How are the analytes measured?
The analytes tend to be measured within a hospital laboratory via a biochemistry analyser, like the one shown alongside. The automatic method is a preferred method because in a hospital there can be hundreds of samples (or even thousands, depending on the size of the hospital) that enter the lab on a daily basis and they may require several tests per sample. This would be very time-consuming if each of the tests had to be done manually, and this is particularly important for urgent samples of patients in the Emergency department.
What do the reference range and critical values entail?
These three values are very important in a clinical setting, as they can help determine the health of the patient if the values are off.
This indicates the range of values seen of a specific analyte (mineral) in healthy people. The reference range can differ occasionally, either due to a pre-existing disease or medication that the patient takes, and therefore the range can end up either higher or lower than the majority (around 95%) of the population. As the Smart Art Graphic alongside shows, what may be seen as healthy in some patients may be interpreted as diseased in other patients, and there is the potential for false positives (i.e. disease where there is none) and false negatives (i.e. diagnosed as healthy where there is disease present).
An example off the top of my head of where medication can change the reference range is
that patients who take heparin (as displayed by the picture alongside) in order to thin their blood would have lower amounts of potassium in the blood than the majority of the population. For the majority of the population, values below 3.8mmolL-1 would be considered as abnormal, however, for patients taking heparin, this level would be perfectly normal as they can have levels as low as 3.1mmolL-1 before they are diagnosed with issues in the blood.
The critical values indicate the level the body can tolerate before it shuts down. For some minerals i.e. sodium, the range at which the body can tolerate is quite wide, however there are some minerals such as calcium which are very tightly controlled and the patient can be in severe trouble if the levels in the blood fall outside these values. However, the results given in the lab are not always accurate, and it is important to check the blood and the clinical history before treating the patient.
Clinical or Artefactual?
In some circumstances, the results may not truly represent what is going on inside the patient. These circumstances are referred to as artefactual results, as they are often caused by the following factors:
- the test tube being in extensive storage
- the wrong tube being used accidentally,
- problems occurring with the analysing machines (systematic or random errors).
There are multiple tubes that a phlebotomist uses in order to put blood in: they each are used for separate tests and have different coloured tops.
A common example of an artefactual value is severely elevated potassium: in some cases, the patient may indeed be having high levels, but more often than not, this is a falsely elevated level. If a test tube of blood lingers in a fridge for more than four hours or the blood is accidentally placed in a K+/EDTA tube instead of a clotting or general-purpose gel tube (i.e. a purple-topped tube instead of a red or white-topped tube respectively), the potassium levels might appear to end up critically high, even when the patient may not even be in any heart distress whatsoever.
How do you prevent artefactual errors?
It is very important, therefore, to cross-check the storage, the type of test tube, the analysing machines and the patients’ clinical history form that arrives with every sample to determine whether the results are truly representative or not. In some cases where the results are extremely high and no obvious artefactual errors can be found, such as a patient who is lipaemic (has extremely high cholesterol levels) the lab may need to perform both reruns on the sample to confirm the high result, and then dilutions on the samples. Once the diluted samples are run through, the necessary calculations are made before the results are sent out.