Common blood tests: Part 1

Blood test tubes

GPs can request a whole host of blood tests which can help to make or exclude specific diagnoses. Blood tests should generally be used for three specific circumstances.

1. To prove a suspected diagnosis where a positive result would change how the patient is managed. In other words, if a result was to come back positive and that would not alter how the symptoms are treated, then it doesn't make much sense checking it in the first place.

2. To exclude a potential diagnosis. For example, a patient with tiredness, having a blood test to exclude anaemia.

3. For monitoring. This can be either to check if treatment is working, or to make sure that treatment isn't causing a known problem, for example checking the kidney function in someone who is on a medication that is known to cause kidney function to worsen.


In the first part of this series, some of the most common Haematology blood test are discussed below:

FBC (Full Blood Count):
This checks Haemoglobin concentration (Hb), average red blood cell volume size (MCV, mean cell volume), along with white cell count (WCC) and white cell count differential, and platelet count (Plt).

- Haemoglobin concentration determines whether a person is anaemic (if too low, which is very common) or polycythaemic (if too high, which is uncommon). Common causes of anaemia are iron deficiency, folate deficiency, vitamin B12 deficiency, anaemia of chronic disease e.g. chronic kidney disease, and blood loss happening over a long time e.g. heavy menstrual bleeding. High haemoglobin could be as a result of long-term smoking leading on to chronic lung disease, or excessive consumption of alcohol.
- MCV can be used to indicate what type of anaemia a person has. For example, if it is low, it could indicate iron-deficiency (commonest cause of anaemia in the UK), and if it is high, it could indicate folate or vitamin B12 deficiency. MCV could also be raised in isolation due to excessive alcohol consumption.

- White cell count (WCC) is usually raised due to an infection, which could be anywhere in the body. If very high (2-3 times normal upper limit), it can point towards something more serious such as cancers affecting the blood (leukaemia). It can also be slightly raised in people taking long-term steroid medication (such as Prednisolone) for conditions like Rheumatoid Arthritis and other inflammatory conditions.
- WCC differential include measurements of the different types of white blood cells, and these can point towards certain diagnoses. For example, raised neutrophils may indicate a bacterial infection, raised leukocytes could indicate a viral infection, and raised eosinophils may indicate an allergic or hypersensitivity response.

- Platelet count shows how effective the body will be at mounting a clotting response. Platelets are one of the principle components used by the body to clot following a cut, along with the formation of fibrin strands. That is why we generally stop bleeding after a short duration of time following a small cut. It may be low in people who take anti-platelet medication such as Aspirin and Clopidogrel, and in those on chemotherapy or those suffering with viral infections, which is why they may bruise easily. Additionally, there are hundreds of uncommon causes, which may require further investigation.


If we are trying to work out the underlying cause of anaemia, we may consider additional blood tests.

This includes Ferritin (a proxy marker for iron stores), Vitamin B12, and Folate, which constitute the commonest deficiencies leading to anaemia.

- Ferritin is a pretty reliable indicator of how much iron is in the blood in nearly all cases. However, it is not always an accurate indicator of iron stores, as it can be markedly raised in any inflammatory condition or process, including infection. This could mean that the Ferritin result might come back normal, when in fact iron levels are low. In such cases, it can be useful to specifically measure iron levels in the blood. This is not routinely done, as it is much more expensive to check and seldom necessary.

- Vitamin B12 is generally low due to dietary deficiency especially in vegetarians and vegans, but could indicate a condition called pernicious anaemia, which is where the person is unable to absorb Vitamin B12 from their diet due a lack of Intrinsic Factor. It can also be a problem in those who have part of their bowel (terminal ileum, last part of the small bowel) removed, usually in those with severe Crohn's disease or cancer, since this is where Intrinsic Factor is found. These individuals will require lifelong Vitamin B12 injections.

- Folate/folic acid can be low due to dietary deficiencies, but can be easily replaced with supplements. It is especially important for those in early pregnancy, and those with certain types of epilepsy, along with those who are on certain medication, e.g. Methotrexate, which is commonly used in Rheumatoid Arthritis. Larger requirements are necessary for those who are obese.


In addition to the above, there are many other combinations of abnormalities, but some of the most common scenarios have been listed for the reader to better understand why blood tests are requested and what the results may mean, if printouts are given by their GP. In the next part of the series, we will go through some of the common Biochemistry blood tests, including kidney function, and electrolyte blood tests.

Disclaimer: the above article denotes the professional medical opinion of the author, and the general advice contained therein should not be used in isolation as personal medical advice. It is always recommended that you see a medical professional such as your own GP for any personal matters.



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