This topic has appeared with increasing frequency in recent exam papers:

• Question 12.1 and Question 12.2 from the first paper of 2019 
• The identical Question 18.1 and Question 18.2 from the first paper of 2015
• Question 22.1 from the second paper of 2016 (a massively elevated serum ferritin level)
• Question 9.2 from the first paper of 2017 (acute phase response)

The best published resources for this topic would have to be this article by Hawkins et al (2014) and a free online guide to laboratory diagnosis of iron deficiency published by Dr Gross at www.iron.sabm.org. The candidate with infinite time resource may be interested in this reporting standards statement from the Australian Royal College of Pathologists. For

In brief, this complex topic can be reduced to a table.

Body iron content

We have about 3.7 grams of iron in our body, painstakingly gathered from iron in our diet. The majority of it (about 2.5 grams) is locked inside the hemoglobin in our blood, being essential to the transport of oxygen. Another few tenths of a gram are found in myoglobin, which also assists in oxygen management.  A remarkably small amount--about 0.02 g--is distributed between the many different proteins that transfer electrons, such as the proteins of the oxidative phosphorylation electron transport chain that create most of our cellular ATP supplies. The rest, about a gram, is stored inside ferritin to fulfill future needs.

Ferritin

• The gold standard for the assessment of iron stores.
• It consists of about 20% iron.
• It is found in all cells, but especially in hepatocytes and reticuloendothelial cells.
• A small amount of it is present in plasma, and this amount is proportional to the total amount. Ferrtiin is therefore a good marker of total body iron stores.
• Hepatic synthesis of ferritin is stimulated by increased iron, and depressed by decreased iron. It is therefore a good marker of iron overload.
• There is no clinical situation other than iron deficiency in which extremely low values of serum ferritin are seen.
• Extremely high ferritin, however, is seen in numerous conditions. It is an acute phase reactant, and so will be elevated in sepsis or any other sort of inflamatory state. It is also elevated in pregnancy. Very high ferritin is seen in haemophagocytic syndrome.
• Ferritin level discriminates between iron deficiency anaemia (low ferritin) and anaemia of chronic disease (high or normal ferritin)

Causes of an absurdly high ferritin level

In Question 22.1 from the second paper of 2016, the college wanted only one specific diagnosis to explain a serum ferritin level of 120,000. Not to be outdone, in  Question 24.1 from the first paper of 2020, the ferritin level was 181,910 IU/L. There is a whole series of possibilities for a high ferritin, and an article by Moore et al (2013) is a good resource for these differentials. The authors sifted though two years worth of iron studies from a major tertiary hospital, and came up with a list of most common conditions (malignancy was the top culprit). Approximately twenty years previously, another group did much the same thing (Lee et al, 1995). The findings generated by these authors have been combined into the list offered below:

• Malignancy
• Iron overload syndromes
  • Hereditary haemochromatosis
• Adult-onset Still's disease
• Systemic juvenile idiopathic arthritis
• Haemophagocytic lymphohistiocytosis
• Chronic inflammatory rheumatological diseases
• Chronically transfused disease states
  • Sickle cell
  • Thalassaemia
• Acute inflammatory states:
  • Sepsis
• Chronic inflammatory states:
  • Chronic advanced liver disease (mainly alcohol-related)
  • Chronic renal failure
  • HIV

All this considered, the only cause of a truly insane ferritin level is still haemophagocytic syndrome. The causes of this rare disease can be found in the review article by Gritta Janka (2008).

Congenital:

• familial HLH (FHLH)
• Chediak-Higashi syndrome (CHS 1)
• Griscelli syndrome (GS 2)
• X-linked proliferative syndrome (XLP)
• The UpToDate article has whole pages listing obscure-sounding loci of mutation.

Acquired causes

• Infectious
  • EBV
  • CMV
  • HIV
  • Leishmania
• Neoplastic
  • Lymphoma, especially T and NK-cell
• Autoimmune
  • SLE
  • Kawasaki disease

Serum iron

• This test measures all serum iron: both the small amount of soluble ionised ferric iron (Fe³⁺) and the transferrin-associated ferric ion.
• A patient's serum iron values may vary 10-40% within a single day
• There is a predictable diurnal variation
• Serum iron is decreased in genuine iron deficiency
• Serum iron is increased in haemolysis, iron overdose, lead toxicity, hepatic necrosis, and haemochromatosis

Transferrin

• Transferrin is the binding protein which carries iron from the liver into the bone marrow.
• It is also a part of the innate immune system, and transferrin molecules are seen in mucosa, where they bind free elemental iron, reducing its availability to invading microorganisms.
• Transferrin decreases in inflammatory states.
• Its level can also diminish in liver disease, nephrotic syndrome, and due to malnutrition.
• A raised transferrin may be a reaction to an iron deficiency state.

Transferrin saturation

• Each molecule is able to carry 2 atoms of iron
• There is normally about 20-45% saturation.
• Transferrin saturation less than 20% usually means there is too little iron (i.e. an iron deficiency state).
• A high (>45%) transferrin saturation suggests there may be an iron overload state such as haemochromatosis (or, that a patient has recently had an iron infusion).

Total iron-binding capacity (TIBC) and unsaturated iron-binding capacity (UIBC)

• UIBC is the amount of "free" transferrin, unassociated with iron.
• TIBC is the sum of serum iron and UIBC
• The TIBC is therefore a surrogate for a transferrin level and these two laboratory tests can be used interchangeably (usually the lab will only report one or the other).
• UIBC represents the capacity to bind "extra" iron; a raised UIBC is associated with iron deficiency anaemia (and a low UIBC represents a state of iron overload, as all the transferrin molecules are saturated)