Blood results made easy

Figure 1.1: Composition of whole blood, serum and plasma

The main disadvantage of using serum is that the blood has already clotted. In other words, a series of metabolic processes occurred after the sample was collected but before the sample was measured. This can lead to errors in measuring such elements as potassium, phosphate, magnesium, aspartate aminotransferase, lactate dehydrogenase, serotonin, neurone-specific enolase and zinc content. The advantage is that serum can be used to measure constituents that would be destroyed or compromised by the anticoagulant chemicals used in the preparation of plasma samples such as serum protein electrophoresis (SPEP).

The main disadvantage of using plasma (blood that has not clotted, due to the addition of anticoagulants) is that the anticoagulants can interfere with certain analytical methods or change the concentration of the constituents being measured. The advantages of using plasma samples include ‘cleaner’ samples that have not undergone the clotting process, time saving and a higher yield (up to 20%).

Different types of anticoagulants are denoted by different-coloured caps on collection tubes. Typical anticoagulants include sodium heparin (green), sodium fluoride (grey), sodium citrate (blue) and EDTA (lavender). Your local healthcare setting will have a tube guide showing the active component of the tube. A clear cap means that neither a clot-activating (serum) nor an anticoagulant (plasma) is present; these clear-capped tubes are often used as discard tubes. The order of draw can be important if multiple tubes are used, to avoid contamination from one tube to the next. Check with your local healthcare setting, but a typical order of draw using the vacutainer system would be:

1Blood culture

2Sodium citrate (light blue) for coagulation testing (PT, INR, aPTT) and D-Dimer

3Serum (red) for LDH, ionised Ca, drugs (phenytoin, theophylline, methotrexate, lithium), vitamin D, parathyroid hormone, osmolality, bone markers, endocrine testing (excluding thyroid)

4Serum separator tube (SST) (gold) for thyroid function (TSH, FT4, T3, cortisol), nutritional markers (gastrin, B12 folate, ferritin), tumour markers (PSA, CEA, AFP, HCG, CA125, CA19.9, CA15.3), immunoglobulins (IgG, IgA, IgM, IgE), electrophoresis, CRP, thyroid Ab, liver Ab, rheumatology Ab

5Plasma separator tube (PST) (light green) for U&Es, LFTs, cardiac enzymes, Ca, Mg, phosphate, uric acid, total protein, amylase, lipids, bone profile, troponin, iron status

6EDTA (lavender) for full blood count (FBC) and ESR, haemoglobin A1c, homocysteine (send on ice and state time), ACTH

7Cross match (pink) for blood transfusion samples

8Fluoride oxalate (grey) for glucose

9Trace element (royal blue) for lithium and magnesium

Haemolysed samples

If, following centrifugation, the plasma or serum looks reddish rather than straw yellow, the sample has probably haemolysed. In a haemolysed sample, some of the red blood cells have lysed (broken open) and their contents will have contaminated the plasma or serum sample. This will cause errors in reporting, for example, elevated potassium, magnesium and phosphate. The laboratory may be able to negate the effect of using a haemolysed sample if the result is needed urgently or it is difficult to obtain another sample. Common reasons for the sample being haemolysed include:

the collection needle gauge being too narrow

over-vigorous shaking of the sample

an underlying haematological disorder

red cells being isolated for storage and then stored in water or a non-isotonic solution

over-vigorous dispensing of blood from the hypodermic syringe to collection tubes.

Reference ranges

Most people are comfortable with the idea of reference ranges, but what do they actually tell us? Or, rather, what don’t they tell us? To create a reference range, a number of volunteers (usually over 120) are matched for factors such as age, gender and ethnicity, and the analyte of interest is then measured. Firstly, most ranges have a 95% confidence, which means that the top 2.5% of values and the bottom 2.5% of values are omitted. In other words, it is possible to be healthy but outside the reference range because you are at the very top or the very bottom, neither of which are shown. Secondly, you should use ranges from unvalidated sources with great care, as ranges can vary with age and gender and local population. Best practice is to use the range that is presented with the actual value.

Storytelling: If you measured the height of 120 shoppers at a supermarket, the world’s smallest man and the world’s tallest man might be present and be included in your sample. However, it’s unlikely that you’ll see them again, so the top and bottom of the data is cropped, leaving more of the average making up the range. This is the first problem with reference ranges. Of the healthy cohort, around 5% are excluded at this initial stage, leaving them outside the range.

Range hangover

As we have seen, reference ranges refer to us on ‘good day’ or peak health. However, most of our patients will be ‘out of range’ so we need a strategy to deal with this. Firstly, let’s consider the idea of range hangover.


Figure 1.2: Range hangover

In Figure 1.2 (above), the x-axis shows the increase in number from left to right. The green line represents a healthy (usually asymptomatic) individual and the red line a patient with a disease (usually symptomatic). The two blue vertical lines represent the reference range. The line on the left is the lowest value and the one on the right is the highest. Because the upper range blue vertical has been drawn in a particular place (called the cut-off value), some of the green line ‘overhangs’. This means that the patient who is healthy may be reported ‘out of range’.

Clinical sensitivity and specificity

Because most blood tests have an associated pathology but may not be very accurate in detecting a specific disease, they sometimes produce ‘false positives’. Clinical specificity refers to whether the test can correctly report someone without the disease as being ‘healthy’. Conversely, clinical sensitivity refers to whether the test can correctly report someone with the disease as being ‘diseased’.

The terms used to describe this are:

True negative (TN): patient is healthy and blood test result is within reference range

True positive (TP): patient has condition and blood test result is outside the reference range

False negative (FN): patient has condition but the blood test result falls within the normal reference range

False positive (FP): patient is healthy but the blood test result is outside the reference range.


Figure 1.3: The cut-off value between ‘healthy’ and ‘diseased’

The cut-off value is the point at which people change from being labelled ‘healthy’ to being labelled ‘diseased’, or the reverse. If we move the cut-off value to the far right, everyone who is healthy will be reported as healthy, but more diseased people will be missed (because they are wrongly labelled healthy). If we move the cut-off value to the far left, everyone who is diseased will be reported as diseased, but more healthy people will be wrongly labelled ‘diseased’. At this point, we need to consider factors such as cost of screening, reliability of data and (most importantly) medical ethics. Is telling someone they have cancer when they don’t as serious as missing someone who does have cancer?

Clinical decision limits (CDLs)

A clinical decision limit (CDL) allows us to prepare protocols and flow charts and consider patient journey management which often informs what we do next.

If we look again at the range hangover diagram (Figure 1.2) but this time add several arbitrary additional cut-off values, we can give each of these a letter and assign typical conditions and actions to each one. (Remember to seek local confirmation as these are only examples to illustrate the theory.)

In Figure 1.3 (below) the disease group is divided into sections A to D, based on the values being 2-fold, 4-fold and 10-fold higher than the upper range.

In practice, if we consider the liver enzyme alanine aminotransferase (ALT) this usually has an upper limit of 40. Our CDLs may therefore look as follows:

‘Normal’ reference range for ALT = (10–40)

Group A = a blood test result for ALT between 40 and 80 (2x the upper normal limit of 40)

Group B = a blood test result for ALT between 80 and 160 (4x the upper normal limit of 40)

Group C = a blood test result for ALT between 160 and 400 (10x the upper normal limit of 40)

Group D = a blood test result for ALT over 400 (greater than 10x the upper normal limit of 40)


Figure 1.3: The cut-off value between ‘healthy’ and ‘diseased’

Strategy for values outside the range

Quite often the patient’s blood test results will fall outside the reference range. As blood test results take the form of numbers and are not binary (like a broken arm), they can be viewed subjectively. It may be helpful to ask yourself the following questions:

1How close is the value to the limits of the reference range? Consider FN, FP, TN, TP, and the mathematical limitations of reference ranges discussed above. This point is about the inherent variance in the range, person, blood sample, and so on.

2Has the test been repeated?

3Is the value increasing or decreasing over time?

4Has the patient had surgery or intervention? An elevated range of inflammatory markers, such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and interleukin 6 (IL6) would be expected post-surgery.

5Does the blood test fit with other ‘indices’ or family markers? Most tests fit into a family group (Hb, RBC and Hct). This point will be further explored in the case study in Chapter 4.

6Is it clinically relevant? The patient may have competing pathologies. Some values will therefore be outside the range because they represent a pathology or condition that is being managed, such as chronic obstructive pulmonary disease (COPD), alcoholism or diabetes. You may be looking for a new out-of-range value or an unusually high value.

7Will it change the patient’s treatment? Before requesting the test, consider what you will do with the result and why you are requesting it.

However, you should bear in mind that the above questions should always be used in conjunction with local clinical procedures in your own healthcare setting.

We may therefore choose to do the following by applying the four key questions we outlined earlier:

Question 1: How far out of range is the result?

Question 2: Does it make sense? Is the patient symptomatic?

Question 3: Do the other family group blood tests agree?

Question 4: Is this an important blood test?

You may choose to construct a decision table.

I have made a start on one below.

Example clinical decision table for the liver enzyme ALT:


We can do this with any blood tests. Let’s try C-reactive protein, which increases with more cellular damage (inflammation). This one is a little trickier, given the timing of CRP and the white cell response (see Chapters 7, 8 and 11).

Example clinical decision table for the inflammatory marker CRP:


There are lots of examples of CDLs being used, from tracking troponin levels in heart attacks to the level of creatinine kinase (CK) in rhabdomyolysis. We then have the extra challenge from the four questions, linked to family groups. We need to ask what we are actually measuring, in order to propose likely actions.

Please note that in these examples we have looked at a pathology which increases blood test values. In patterns of anaemia, for example, the cut-off groups A–D would be less than normal.


1.Name two components of the blood. Can you give an example of each type?

2.List four ways in which the blood can change, using words that make sense to you.

Can you list some examples for each type of change?

3.What are the four questions you need to consider when reviewing a blood test?

4.Using symptoms and blood test results, define ‘false positive’ and ‘false negative’.

Can you give any examples?

Quick reference glossary

The following table shows common terms, abbreviations and some typical observations relating to the various blood tests. Some examples also have metaphors, shown in quotation marks, to aid memory and understanding. These will be explained further in the corresponding chapters.

Table 1.1: Glossary of terms used in blood tests

Full blood count (FBC) – measures haematology of cells

Cell that causes the blood to clot

Also a marker of bone marrow function

Decreased in some leukaemia and myelomas

Additional test is mean platelet volume (MPV)

Low (usually less than 150 10*9/L) is called thrombocytopenia; platelets are decreased in bleeding, liver disease, pregnancy, infection and due to medication

Raised (usually greater than 450 10*9/L) is called thrombocytosis; platelets are raised in infection, inflammation, trauma, post operation or due to medication

White blood cell count (WBC)

The total number of white cells in the blood


A type of white blood cell

Responds to tissue damage via C-reactive protein (or CRP)

Raised in bacterial infections, autoimmune conditions

‘The fire engine’


A type of white blood cell

Makes antibodies

Raised in viral infections and some myelomas

‘The police’


A type of white blood cell

Infiltrates the tissue in systemic bacterial infections

Linked to cardiovascular disease and high low-density lipoprotein (LDL) cholesterol

‘The miner’


A type of white blood cell

Important in allergic responses and hypersensitivity


A type of white blood cell

Important in allergic responses and hypersensitivity


A type of dysfunctional white cell

Raised in leukaemia and myelomas

Haematocrit (Hct)

Percentage of red blood cells in the whole blood

Decreased in anaemia

Elevated in polycythaemia

Haemoglobin (Hb)

The oxygen-carrying protein in the red blood cell

Decreased in anaemia

Elevated in polycythaemia

Red blood cell count (RBC)

The total number of red blood cells in the blood as a count

Decreased in anaemia

Elevated in polycythaemia

Mean cell volume (MCV)

The average size of the red blood cells

Low in iron deficient anaemia

Normal in blood loss anaemia

High in folate and B12 deficient anaemia

Inflammatory markers – measures biochemistry
Plasma viscosity (PV)

A measure of more ‘stuff’ in the blood

Thus, a surrogate, non-specific marker of inflammation

Increased in autoimmune conditions, infection, cell damage, cancer, myelomas

‘The traffic jam due to fire engines and police (white cells)’

Could remain raised for two weeks post-injury, as increased white cells have around two-week lifespan

Erythrocyte sedimentation rate (ESR)

How quickly red blood cells fall in a tube, in a lab

A surrogate, non-specific, marker of inflammation that has elicited a fibrinogen response

Fibrinogen ‘sticks’ red blood cells together so they become heavier and fall more quickly

Could be normal in low damage inflammation as seen in some autoimmune conditions

‘The scaffolding and building-supporting structure following a large fire’

If raised, could remain raised for a significant time post-event

C-reactive protein (CRP)

A chemo-attractant protein released in response to tissue damage

‘The fire alarm’

Possible to miss the CRP response post-injury whilst still having raised PV and ESR

Increasingly being used as a sensitive marker for atherosclerotic vascular damage to indicate cardiovascular risk

Urea and electrolytes (U&Es), Kidney function – measures biochemistry
Sodium (Na)

Extracellular electrolyte that controls water balance and blood pressure

Raised in dehydration, image urea

Potassium (K)

Intracellular electrolyte, controls cellular pumps and receptors via electric potential

Therefore a red flag if in high concentrations in the blood


A marker of acute renal dysfunction, such as distress, although this could be something like dehydration, so image to Na levels


A marker of chronic renal function, such as a renal stone

Estimated glomerular filtration rate (eGFR)

A general marker of kidney function

Used to diagnose chronic kidney disease staging

Used to confirm renal dysfunction as cause of other conditions such as renal anaemia

Liver function tests (LFTs)– measure biochemistry
Alanine aminotransferase (ALT)

A liver enzyme

Often raised in trauma, drug toxicity, and viral hepatitis

Aspartate aminotransferase (AST)

A liver enzyme

Often raised in trauma, acute alcohol hepatitis and liver failure

Also found in the heart so image to cardiac markers/chest pain

Gamma-glutamyl transferase (GGT)

A liver enzyme

Often raised following alcohol intake

image to RBC, MCV and folate to differentiate between alcohol, B12 and diabetes neuropathies

Alkaline phosphatase

A liver enzyme

Often increased in biliary tree damage such as gallstones Also found in the bone (check Ca), kidney (check U&Es) and placenta (check age and gender)


A liver enzyme

Often increased in pancreatitis and pancreatic tumours


A marker of the ‘plumbing’ of the liver

Increased in jaundice, usually caused by pre-, actual or post-hepatic blockage


A bilirubin breakdown product, usually absent in post-hepatic jaundice


A protein produced by the liver

A chaperone for chemicals like Ca so could give false low value in nutrient-deficient patients

Decreased in liver damage


A crude marker of antibody production/presence

Often increased in autoimmune conditions, myelomas and viral infection

Additional tests – measure biochemistry

A breakdown product of a clot

Care should be taken to link D-dimers to deep vein thrombosis (DVT) and pulmonary embolism (PE)

Refer to NICE guidelines (UK)

International normalised ratio (INR)

This is a marker of blood clotting

How long it takes for your blood to clot is given a baseline value of 1, thus an INR of 2 would mean your blood is taking twice as long to clot

Goes up in anti-coagulation and liver disease

Bence Jones protein

A breakdown product of a ‘nonsense’ antibody

Usually present in a myeloma

Bone profile

Usually returns corrected Ca, PTH and vitamin D3

Can help differentiate between osteomalacia (rickets), Paget’s and osteoporosis

Prostate specific antigen (PSA)

Released by the prostate

Relative to prostate damage

A slightly raised PSA may not mean prostate cancer

Link to urea and Alk phos, and Ca (secondary bone metastasis)


A marker of ovarian cancer

Thyroid function

Thyroxine (T4), thyrotrophic releasing hormone (TRH) and thyroid stimulating hormone (TSH) are measured to diagnose cause of primary, secondary or tertiary hypothyroidism (or hyperthyroidism); additional confirmatory tests may be required

Also used to titre T4 supplements

Autoimmune markers

Rheumatoid factor (rheumatoid arthritis) and ankylosing spondylitis (HLAB27) are types of self-antibodies that are often present in autoimmune conditions

Intrinsic factor and parietal cell antibody for pernicious anaemia

Haemoglobin with glucose irreversibly bound (HbA1c)

A long-term marker of glucose in excess, used in diabetes monitoring

Acid Base pH Bicarbonate

Used to monitor respiratory (chronic obstructive pulmonary disease) and metabolic (drug overdose) acidosis or alkalosis

Ferritin (Iron), Folate, B12

Nutrient markers to be used with RBC, Hb and MCV

Low MCV usually has low ferritin (high ferritin in hemochromatosis)

High MCV usually has low folate and/or low B12

Low folate from sustained alcohol, drug interactions, diet or gastro-intestinal (GI) conditions

Low B12 from GI conditions, diet and autoimmune pernicious anaemia

Troponins, creatine kinase mb (CKmb), B-type natriuretic peptide (BNP)

Cardiac event markers

Troponins and CKmb are proteins found in the cardiac tissue that are present in high concentrations in the blood following a cardiac event

BNP is a peptide found in the cardiac wall; increased levels may mean ventricular wall load is dysfunctional and may predispose to a cardiac event

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Mar 13, 2020 | Posted by in HEMATOLOGY | Comments Off on Blood results made easy

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