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Blood tests

Contents

  1. General screening tests
  2. Immunology
  3. Endocrinology
  4. Miscellaneous

There are hundreds of blood components that can provide information about all kinds of metabolism, bodily functions and malfunctions. Only a small number of blood tests are routinely carried out. The following is a selection of some of the more commonly used blood tests. The selection is subdivided into:

·      general screening tests (general screening)

·      tests that address immune-system related questions (immunology)

·      hormones and other substances produced by various glands (endocrinology)

·      some other tests (miscellaneous).

We do not quote quantities or ‘typical’ results for any of the tests, for three reasons: i) some test results are reported as relative rather than absolute quantities and depend on individual laboratory procedures and calibration routines; ii) depending on individual circumstances a deviation from the standard range of quantities may not be indicative of any disorders; iii) for a particular condition only a combined set of results from different tests may give meaningful information.

Looking for so called tumour markers is commonly included in the diagnosis and monitoring of a number of malignancies by blood tests. However, tumour marker tests are irrelevant for head and neck malignancies and thus are not included in the following selection of blood tests.

General screening tests

FBC, full blood count

A so called full blood count is a common general screening test. It analyses the composition of blood in terms of concentration of red blood cells (oxygen transport around the body), haemoglobin (the specific molecule binding oxygen molecules for transport), white blood cells (part of the body’s immune system and necessary for fighting infections) and platelets (part of the blood clotting system).

Abnormally low concentrations of haemoglobin can indicate anaemia or some form of chronic bleeding; abnormally high concentrations of haemoglobin can give a hint at bone marrow problems.

Abnormally low concentrations of white blood cells may be related to bone marrow problems or viral infections; high concentrations of white blood cells are a sign of an ongoing infection or may occasionally be related to a haematological malignancy (blood cancer).

Low concentrations of platelets may be caused by viral infections or some autoimmune diseases, by medications (including a number of chemotherapy drugs), by vitamin K deficiency or some blood malignancies such as leukaemia. High platelet concentrations are most commonly a sign of inflammation or infections, occasionally indicative of a bone marrow disorder.

Electrolytes

Testing the levels of electrolytes is a common part of a general assessment. An electrolyte test measures the levels of Na+, K+, Cl- and HCO3- (bicarbonate) and indirectly determines the pH balance (acidity level) as well. These electrolytes are necessary to regulate the body fluid retention (water), the body’s response to electrical signals (heart beat), transport mechanisms in and out of cells across cell membranes, and stabilisation of pH.

An electrolyte test is not specific to a particular condition, many disorders can cause electrolyte imbalances, for example dehydration, kidney disorders or heart conditions, or abnormal levels of some hormones. An electrolyte test is an informative first step in the investigation of fluid retention (oedema) or irregular heart beat (dysrhythmia).

Glucose test

This is a simple  screening test where the blood glucose level is measured once, either after several hours fasting or randomly. Higher than normal glucose levels for either circumstance will need to be followed up by a more detailed glucose tolerance test (see below under endocrinology). The diagnostic usefulness of a single glucose test is questionable unless it is done as part of the structured and regular self-monitoring routines of people with diabetes.

ANA, autoantibody screen

This test is an initial search for the presence of a particular antibody, antinuclear antibody (ANA). This antibody is often present in a range of autoimmune diseases such as rheumatoid arthritis, Sjögren’s syndrome, lupus erythematosus, or some forms of hepatitis. If this antibody is found, in a next step screening for more disease-specific antibodies usually follows in order to arrive at a diagnosis.

PT / INR test

A prothrombin time (PT) test measures the time it takes for blood to clot. In order to make data from this test comparable between laboratories and for different test methods, the data are usually reported as relative quantities, known as the international normalised ratio (INR).

The PT / INR test is used to monitor the working of long-term treatment with so-called blood thinners (warfarin) and to check for bleeding risks in planned surgery. Abnormally long coagulation times can be indicative of liver disorders (some of the coagulation factors are produced in the liver), or lack of vitamin K (necessary for the synthesis of prothrombin and other clotting factors). Low levels of coagulation factors, causing hard-to-control bleeding and bruising, are also found in hereditary conditions (haemophilia).

A PT / INR test is often followed up by further coagulation tests and/or liver function tests (see below).

Coagulation tests / clotting studies

The coagulation (clotting) properties of blood need to be finely balanced: coagulation must be easy enough to occur so that bleeding after injury stops promptly; at the same time coagulation must not occur in normal blood circulation because blood clots would kill the body very quickly.

Conditions causing clotting abnormalities include liver disease (many proteins involved in coagulation are produced in the liver), haemophilia (lack of coagulation) or thrombophilia (excessive coagulation).

A set of different tests aims to characterise the causes of coagulation disorders by measuring the levels of several proteins and other components in blood and/or the time it takes for blood to clot. Coagulation tests are also necessary to monitor the working of long-term treatment with anticoagulants (warfarin or heparin) and may be required before surgery to minimise bleeding risks (modern synthetic anticoagulants do not need monitoring but their action cannot be reversed).

Sets of coagulation tests may include:

FBC; full blood count (see above).

PT / INR; prothrombin test (see above).

Platelet count; is usually included in a FBC (see above).

Thrombin time, TT; this test is an indirect assessment of the function of the protein fibrinogen. Long thrombin times can be due to either low levels of fibrinogen or abnormal fibrinogen function, or both. Long thrombin times are also found in conjunction with malnutrition and advanced liver disease.

Fibrinogen level; this test determines the level of fibrinogen in blood plasma. Enhanced levels occur with acute inflammation (for example, in the early stages of periodontal disease) whereas low levels can indicate liver disease, or a condition where fibrinogen is consumed faster than it is synthesized (disseminated intravascular coagulation, DIC).

Factor V level; factor V is a protein, predominantly synthesized in the liver; in its activated form it functions as a co-factor in the enzymatic reaction that converts prothrombin to thrombin. Low levels of factor V cause prolonged PT times and may indicate liver disease or DIC; normal levels of factor V may be hampered by interaction with some antibodies.

Cholesterol and triglycerides

Determining the levels of cholesterol (and triglycerides) in the blood is a common test to assess the lipid metabolism of the body.

Triglycerides are the fat-storage lipid of the body, they are produced in the liver and are predominantly the end product of digesting fat in food. High blood levels of triglycerides have been related to increased risk of heart disease.

Cholesterol is a transport form of lipid in blood for delivery to all body tissues; cholesterol is a necessary component in the cell metabolism. The transport form of cholesterol is called a lipoprotein, with cholesterol bonded to a protein. There are two types of lipoproteins in circulation: LDL (low-density lipoprotein, ‘bad cholesterol’) and HDL (high-density protein, ‘good cholesterol’). High levels of LDL are thought to be responsible for lipid deposits in arteries, reducing the efficacy of blood flow and increasing the risk of heart attacks and stroke. HDL is the form in which excess cholesterol is transported back to the liver for processing.

High levels of triglycerides and/or cholesterols may be related to poor diet and/or lack of exercise, or may be caused by genetic conditions. An accurate cholesterol and triglyceride test requires at least 12 hours fasting before the test.

Folate / folic acid

Folic acid belongs to the class of B vitamins. Folic acid is essential for the maintenance and renewal of blood components (red and white blood cells, platelets) and for the normal development of a foetus.

Folic acid levels measured in blood plasma depend on recent dietary intake whereas the folic acid levels in red blood cell reflect an average stored amount over a period of time.

Folic acid levels are often measured together with levels of vitamin B12, as low levels of either of these two B vitamins can cause anaemia. Anaemia may also be caused by malabsorption of folic acid or vitamin B12.

Immunology

ASDA test

This test looks for the presence of specific salivary gland duct antibodies in blood. These are a marker for Sjögren’s syndrome, an autoimmune disorder leading to inflammation of some glands and, in consequence, especially to dry mouth and eyes from lack of tear and saliva production.

The test returns a positive finding only for about two thirds of cases of Sjögren’s syndrome and further tests may be necessary to diagnose or exclude this rare condition.

C1-INH test

C1 is one of nine very large protein molecules (C1 to C9) that circulate in the blood stream and are part of the body’s immune system. Testing the levels of C1 in blood is one (of many) test to help identify specific causes of inflammation and swelling (oedema). Abnormal concentrations of C1 are in particular indicative of a condition called angioedema and of a number of autoimmune disorders (for example, lupus erythematosus). Low levels of C1 are associated with kidney diseases (nephritis), septicaemia and recurring bacterial infections.

Allergy tests, IgE

Testing for allergies is most commonly done by skin tests. However, if severe eczema or other serious skin conditions prevent skin testing, or if a severe allergic reaction (anaphylactic shock) is expected from exposure to allergens during testing, then using blood samples to look for allergy markers is an alternative. A high concentration of a particular type of antibody, immunoglobulin E (IgE) tends to indicate allergic reactions. When high levels of IgE are found, further screening for several of the most common allergens (such as dust, moulds, specific foods) is usually carried out by searching for allergen-specific antibodies. This screening approach is called enzyme-linked immunosorbent assay (ELISA).

Measuring blood levels of IgE may also be included in a wider screening of different types of antibodies, in immunoglobulin tests (see below).

Immunoglobulin tests

Immunoglobulins, Igs, are antibodies found in all areas of the body. Antibodies are the frontline of the body’s immune system, defending the body against infections and other foreign substances. Antibodies also represent a body’s memory with regard to successfully overcome infections in the past, by keeping copies of antibodies acquired in the process and which can be re-activated should the need arise (this being similar to the underlying mechanisms of immunisation by vaccines). The roles of antibodies are varied, accordingly there are several different categories of antibodies.

IgG. The most common antibodies (ca. 75 percent), found in all body fluids. IgG antibodies are crucial in combatting bacterial and viral infections.

High levels of IgG are related to chronic infections (such as HIV or hepatitis), multiple sclerosis and multiple myeloma where only one type of defective IgG antibody is produced. Low levels of IgG are observed with some types of leukaemia, kidney damage and recurrent infections. Macroglobulinaemia (abnormally high levels of IgM) suppresses the synthesis of IgG.

IgA. The second most common type of antibodies (ca. 10-15 percent) they mostly protect inner and outer body surfaces (nose, eyes, vagina, digestive tract) and are found in saliva, tears and blood.

High levels of IgA are related to chronic liver diseases, a range of autoimmune disorders and some type(s) of multiple myeloma. Low levels of IgA are observed with some types of leukaemia and with kidney damage (nephrotic syndrome).

IgM. The third most common type of antibodies (ca. 5-10 percent) and very large molecules. They are found in blood and lymph fluid and are the first-line defence response to infection, as well as regulating the responses of other antibodies.

High levels of IgM are observed in acute new infections, especially viral infections (hepatitis, mononucleosis), in rheumatoid arthritis and macroglobulinaemia. Low levels of IgM are related to some forms of multiple myeloma and some forms of leukaemia, as well as some autoimmune disorders.

IgE. IgE antibodies specialise in triggering body reactions to foreign substances such as pollen or mould spores. Often high IgE levels are found in connection with allergic reactions (see above; allergy test IgE) and asthma.

IgD. IgD antibodies are found in relatively small amounts in the lining of the chest. It has been speculated that they share some of the role of IgM or, more recently, it has been suggested that IgD antibodies have specific roles in respiratory immune defence of the respiratory mucosa.

Typically, Ig tests include screening the levels of IgG, IgA and IgM. Combined consideration of the results for all three types of antibodies is useful to identify certain autoimmune diseases, some haematological malignancies, reasons for recurring infections and bone marrow disorders.

Specific antibody tests are needed to check the response to immunisations and to identify the presence of specific infections, past or present.

Endocrinology

Glucose tolerance test

This procedure monitors how efficiently the body processes glucose. Glucose is the sugar that the body uses as an energy-provision molecule. Persistent high blood levels of glucose (hyperglycaemia) are indicative of diabetes and a few other conditions. Low blood levels of glucose (hypoglycaemia) most commonly afflict people with diabetes on insulin when, for example, they get their timing and quantity of insulin injections and eating and/or quantities of food wrong. Otherwise hypoglycaemia may be related to insufficient function of the adrenal gland or pancreas, or to various liver or kidney disorders (substantial hypoglycaemia is a dangerous condition and needs immediate intervention).

A blood sample is taken first and the amount of glucose in the sample is determined. Next a measured quantity of glucose is given, either orally (as a very sweet drink) or directly as an injection into a vein. Then a few more blood samples are taken at intervals and the glucose blood levels are monitored as a function of time. Slower than normal removal of glucose from the blood stream can indicate diabetes.

The actual amounts of glucose that normally circulate in the body are small and only fluctuate little in a healthy person. The most accurate methods to measure these small glucose concentrations are by reacting a small amount of blood with a calibrated amount of a glucose-specific enzyme (glucose oxidase or hexokinase). These are usually prepared on a test strip.

HbA1c test

Haemoglobin (Hb) molecules are the proteins in the red blood cells responsible to transport oxygen around the body. If glucose is present in the blood, glucose molecules will firmly bind to the Hb protein molecules, forming a stable complex. This form of ‘glycated’ Hb protein molecules is called HbA1c. The amount of HbA1c complex is directly proportional to the glucose level in blood.

Measuring HbA1c levels in blood is an important component in diabetes monitoring. A glucose tolerance test (see above) gives a measure of the on-the-day performance of the body with regard to glucose processing / insulin availability, something like a snapshot. The HbA1c test provides insight into average blood glucose levels over a longer period of time, over ca. 2-3 months. The test provides information about average glucose levels over that period of time because the Hb and HbA1c carrying red blood cells survive for about 2-3 months in the body. This long-term monitoring is an important tool in trying to avoid or minimise long-term harm caused by poorly controlled glucose levels in diabetes.

Short synacthen test

This test measures the levels of the hormone cortisol in blood, after stimulation of its production. Cortisol (and the proper regulation of its production) is important for the functioning of, amongst others, the immune system and the regulation of blood pressure and insulin levels in the blood.

The production of cortisol (‘on demand’ production) is regulated by a number of interconnected feedback loops involving the hypothalamus (monitoring body functions), the pituitary gland (initiating cortisol production by releasing a hormone, ACTH) and the adrenal glands (next to the kidneys) where the actual cortisol production is triggered by ACTH.

The short synacthen test involves the injection of a chemical, tetracosactide, (synacthen) that activates cortisol production in the adrenal glands, just as ACTH does naturally. If after this stimulation cortisol levels in the blood remain low, this can indicate a failure of the adrenal glands (for example caused by a rare autoimmune disease, Addison’s disease). If the cortisol levels in the blood increase after the stimulation, this may indicate damage to, or malfunction of the pituitary gland (for example caused by a tumour or damage caused by radiotherapy applied to the head and neck region).

Amylase

Amylase is an enzyme that is found in saliva, it is synthesized in the salivary glands and the pancreas. The physiological role of amylase is to help with the break-down and digestion of starch. High concentrations of amylase in blood (or urine) indicate acute inflammation of the pancreas (pancreatitis) or severe gastroenteritis.

TSH test

A thyroid stimulating hormone, TSH, test is carried out as a part of checking the thyroid gland and its role in a complicated chain of events of hormone releases. A small structure in the brain, the hypothalamus, releases a hormone TRH (thyrotropin-releasing hormone). TRH activates the pituitary gland (hypophysis, another small structure in the brain, close to the hypothalamus) to produce TSH, and finally this release of TSH activates the thyroid gland to produce and release two hormones (triiodethyronine and thyroxine), both being an integral part of the body’s metabolism control mechanism.

A TSH test helps to find out if over- or underactivity of the thyroid gland is due to a damaged gland or due to problems elsewhere in the chain of hormone-release events. TSH testing is also used to monitor medication for hypothyroidism (underactive thyroid gland) or for replacement medication after removal of the thyroid gland.

High blood levels of TSH indicate an underactive thyroid gland (with Hashimoto’s thyroiditis being the most common cause), very rarely a pituitary gland tumour (producing TSH), or simply identify poorly adjusted medication for treating hypothyroidism. Low blood levels of TSH are related to an overactive thyroid gland, occasionally to damage of the pituitary gland, or simply to poorly adjusted medication for treating hypothyroidism.

PTH test

Measuring the parathyroid hormone, PTH, levels in blood is an investigation into the function of the four parathyroid glands (tiny glands in the neck, close to the thyroid gland). PTH is essential in the regulation of bone growth and maintenance. PTH controls vitamin D, Ca2+ and PO43- levels in blood. Accordingly, a PTH test provides information about severe forms of osteoporosis, about the reasons behind high Ca2+ and low PO43- blood levels, and about certain kidney disorders.

High levels of PTH are found for several disorders of the parathyroid glands, lack of vitamin D, some kidney disorders, the body being immune (unreactive) to PTH, and poor diet (lack of Ca2+). Low levels of PTH are found after removal or damage of the parathyroid glands in neck surgery, or damage of the parathyroid glands by some autoimmune disorders or by radiotherapy to the head and neck region. Bone metastases from malignancies elsewhere in the body, low Mg2+ blood levels, excess vitamin D and/or Ca2+ intake (supplements) also lead to low PTH blood levels.

Miscellaneous

ESR test (inflammation indicator)

The erythrocyte sedimentation rate (ESR) is a measure of the severity of the body’s response to inflammation and can be used to monitor inflammatory conditions; it does not identify the cause of inflammation.

Inflammation alters the surface structure of red blood cells (erythrocytes), causing them to build aggregates of red blood cells. This tendency to form aggregates is greater the more severe the body’s reaction to inflammation is.  Aggregates of red blood cells settle more quickly to the bottom of a tube than individual red blood cells. Accordingly, this sedimentation rate is an indirect measure of the severity of the body’s immune system response.

CRP test (inflammation indicator)

C-reactive protein (CRP) is produced in the liver as part of the response of the body to inflammation, it is not specific to a particular cause of inflammation. The role of this protein is to enable macrophages to dispose of dead or dying cells (including some bacterial cells). A high level of CRP in blood plasma is an acute-phase indicator of inflammation, similar to ESR (see above).

TPMT test (drug safety)

Thiopurine methyltransferase, TPMT is an enzyme needed for metabolising thiopurine compounds. Absence or low activity of this enzyme is a fairly common genetic deficiency (affecting ca. 1 in 300). Testing for TPMT activity is necessary to find out if treatment with azathioprine (and similar drugs) is safe or carries a risk of severe side effects (such as bone marrow suppression) if not properly metabolised due to lack of enzymatic activity. Azathioprine (and similar drugs) belongs to the class of so called antimetabolites. It is used as an immunosuppressant in the treatment of a range of autoimmune diseases (such as rheumatoid arthritis, Crohn’s disease or lupus erythematosus).

Absence or low activity of TPMT also acts as an indirect biomarker for predicting the toxicity of and/or intolerance to chemotherapy treatment with cisplatin [treatment-chemotherapy-chem-principle-level1].

Zinc (trace element)

Zinc, that is Zn2+ ions, is an essential trace element for a variety of body functions; it is a co-factor in many enzymatic reactions. The Zn2+ concentration is usually measured in blood serum or plasma.

A lower than normal Zn2+ concentration in blood can occur as a consequence of excessive alcohol consumption, of serious burns or of a poor diet. Zn2+ deficiency causes problems with wound healing, can cause a range of skin conditions and hampers the immune system.

Magnesium (electrolyte)

Magnesium, that is Mg2+ ions, are a part of the overall electrolyte orchestrated ‘traffic’ into and out of cells. Mg2+ ions are necessary for muscle and nerve function as well as for the activity of some enzymes. Only very small amounts are in circulation, most of it is found in bone and inside cells. Mg2+ blood levels are closely related to the blood levels of other electrolytes (Ca2+, K+, Na+, PO43-). One purpose of testing Mg2+ blood levels is actually to find out about underlying reasons for abnormal blood levels of other electrolytes, in particular Ca2+.

High Mg2+ blood levels can indicate kidney failure, dehydration, malfunction of the adrenal glands (Addison’s disease), an underactive thyroid gland, or simply occur as a consequence of (over)use of Mg2+ containing over-the-counter drugs (antacids and laxatives).  Low Mg2+ levels are rare and mainly are related to diabetes complications, kidney disorders, or pancreatitis. Low Mg2+ levels are also found in conjunction with excessive alcohol consumption, or alcohol withdrawal, as well as a poor diet.

Trace metal ions

Trace metal ions are present in very small but measurable quantities in cells. They include ions of iron, zinc, magnesium, lithium, nickel, cobalt, vanadium, copper, chromium, manganese, selenium (not a trace metal but a trace element). Many ions of these elements would be toxic in higher concentrations but are essential components in very low concentrations as components in numerous enzymes and vitamins, usually involved in specific biochemical reactions. Sources of trace metal ions are usually dietary.

Insufficient levels of trace metal ions can cause a wide variety of deficiency effects, such as low iron levels and anaemia. Excessive intake of trace elements, for example by overuse of dietary supplements, can cause toxicity effects.

Urea (kidney function)

Urea is a metabolic waste product that is synthesized in the liver, from there it is carried by the blood stream to the kidneys. Healthy kidneys will filter out this waste product from the blood and urea consequently is excreted via urine. High urea levels in blood indicate poor kidney function for a number of possible reasons, or may be related to a number of other conditions, ranging from severe burns to heart failure. Low urea levels in blood are rarely found, except in pregnancy or as a consequence of a poor diet of vegetarians when their diet is too low in proteins.

A urea blood test can be indicated for diagnosing any of the conditions mentioned above, it is commonly used to monitor dialysis patients and the treatment of kidney disorders in general. In addition, it is used to assess kidney function before starting treatment with drugs that may be nephrotoxic (harmful to the kidneys), such as chemotherapy treatment with cisplatin.

Creatinine (kidney function)

Similar to the urea test (see above), a creatinine test measures the ability of the kidneys to remove creatinine from the blood by determining the creatinine levels in blood serum. Creatinine is another waste product, originating from muscle metabolism, and will be efficiently filtered out by healthy kidneys. The purposes of creatinine tests are similar to those of urea testing but include the monitoring of some muscle disorders, diabetes and high blood pressure.

It is common to combine urea and creatinine tests, and there may be reasons to measure in addition the amounts of urea and/or creatinine in urine.

BIL (liver function)

Bilirubin (BIL) is the waste product from breaking down old red blood cells. BIL is a yellow pigment, high levels of bilirubin lead to jaundice (yellow skin and white of eyes). Bilirubin is transported to the liver and then stored in the gallbladder, then in the form of bile helps to digest fat in the small intestine, before being excreted with the faeces.

High bilirubin blood levels can be caused by liver conditions (such as hepatitis or cirrhosis) when the liver fails to process normal quantities of bilirubin, gallbladder conditions or blockages of the bile ducts. Alternatively, if a larger than normal quantity of red blood cells are broken down (haemolysis) the (healthy) liver may not be able to keep up with bilirubin processing. This may be caused by some autoimmune conditions, infections or drug toxicity.

A bilirubin test is usually a part of a more general set of liver function tests (LFT, see below) and is also commonly used to assess the safety of drug treatments with high potential for liver toxicity.

LFT, liver function test

A liver function test (LFT) is a set of several blood tests, measuring a range of different metabolic processes in the liver. The liver is not only a physically large organ, it also has a large range of metabolic roles in the body.  The tests provide an overview of  liver functions; help to diagnose, and assess the severity of, liver disorders; serve as a precaution before and after starting medications with potential for liver toxicity; and to identify potential liver disease (for example, after exposure to a hepatitis virus, or alcohol dependence).

The set of LFT tests typically includes testing for:

BIL; measuring bilirubin levels (see BIL test above)

ALT (alanine aminotransferase); ALT is an enzyme mostly found in the liver that catalyses some basic steps in protein processing. High ALT levels are a marker for liver disease and damage and help to distinguish the causes for jaundice (liver or blood disorder).

AST (aspartate aminotransferase); AST is an enzyme found in several body tissues such as red blood cells, liver, muscle tissue, pancreas and kidneys. AST catalyses some basic steps in the amino acid metabolism. High AST levels are indicative of acute liver damage (or damage to other tissues).

ALP (alkaline phosphatase); ALP is an enzyme that catalyses removal of PO43- units from molecules and thus is part of the energy metabolism pathways. ALP is mostly found in the liver and in bone. High blood levels of ALP occur with a number of bone conditions (for example, Paget’s disease or bone metastases), with over-activity of the parathyroid glands, lack of vitamin D, and liver damage (including liver metastases). Accordingly, ALP test results have to be interpreted in conjunction with results from other test, in particular ALT and AST.

Albumin; albumin is the most common protein circulating in blood plasma; it is mainly synthesized in the liver. Albumin binds to many different substrates, cell surfaces, water molecules, Ca2+, Na+, K+, hormones, bilirubin, thyroxine which is necessary for its central role in regulating the osmotic pressure of blood (which in turn is essential for the transport properties of blood). Low levels of albumin occur with liver disease, malnutrition and some malignancies; high levels of albumin are usually related to dehydration.

Ferritin (iron imbalance)

Ferritin is a very large protein molecule that acts as an iron storage unit in cells and it releases iron ions as needed and normally maintains a balance between iron over- and underload. Ferritin is a universal protein in nearly all living organisms, to all of which free Fe ions would be toxic.

Increased levels of ferritin in blood can indicate iron overload disorders (such as haemochromatosis), liver disease and also infections or malignancies (the toxins released in the presence of these conditions upregulate the ferritin production).

Low levels of ferritin in blood serum are a specific marker for iron deficiency anaemia. Other conditions leading to low ferritin levels include low activity of the thyroid gland, lack of vitamin C, coeliac disease; vegetarians often have low ferritin levels.

Care needs to be taken in interpreting the results of a ferritin test, given the opposing trends in serum ferritin levels from conditions that may be present simultaneously.