Acute Phase Proteins

Summary

• Many potential acute phase proteins might be tested
• They increase in response to insults such as infection, parasitism, tissue damage or neoplasia
• Main ones in equids comprise:

• SERUM AMYLOID A (SAA)
  • responds more rapidly than other acute phase proteins (within 8-12 hours).
  • continued increases indicate an active inflammatory process and recommendation to continue treatment, or at least monitoring.
  • simpler horse-side SAA analysers produce inconsistent results and are less able to accurately detect changes over time.
• SERUM IRON
  • Serum iron decreases in response to inflammation
  • Changes occur more rapidly than fibrinogen and is significant by 16 hours of the inflammatory stimulus.
  • may not be reliable in young foals and it may also increase in response to general anaesthesia.
• GLOBULINS
  • most of the acute phase proteins are included in the globulin fraction.
  • tends to follow a more chronic time course and is slow to normalise following recovery from inflammation.
  • also common with hepatic insufficiency.
  • serum protein electrophoresis very rarely provides further useful diagnostic or prognostic information unless a severe monoclonal hyperglobulinaemia is present (e.g. myeloma).
• ALBUMIN
  • Serum albumin is a “negative acute phase protein” as albumin levels can fall slightly with inflammation (especially in chronic cases)
  • Tends to be relatively mildly affected and very rarely lower than 25 g/L.
  • (Marked hypoalbuminaemia (<20-25 g/L) is generally indicative of proteinlosing enteropathy (e.g. parasites, NSAID use, IBD etc..)

The acute inflammatory response arises due to insults such as infection, parasitism, tissue damage or neoplasia, and results in a widespread and complex cascade of cytokine and lymphokine production (interleukins, interferons, eicosanoids etc). “Acute phase protein” is
the collective term for proteins which are synthesised and released from the liver in response to inflammatory cytokines, especially interleukin 6 (IL-6). These proteins include serum amyloid A, hepcidin, caeruloplasmin, fibrinogen, procalcitonin, C-reactive protein,
haptoglobin, activin A and several others. Although fibrinogen has been popular for many years, its continued use is now questionable given far better alternatives and increasing problems with reliability and validation of available fibrinogen assay methods. The LEH Laboratory offers serum amyloid A and serum iron as the main diagnostic acute phase proteins which are more sensitive indicators of inflammation than fibrinogen and may be more useful in monitoring responses to infection and other inflammatory challenges such as infection, parasitism, neoplasia, tissue damage and immune mediated disease.

Although the knowledge about iron as an inflammatory marker in horses is not new, Liphook were the first laboratory in the UK to use it as a routine inflammatory marker with the confidence that it is giving us more dependable information than fibrinogen. Iron is an essential element for many microbial processes, and mammalian immunity has evolved to reduce iron availability during inflammation by internalising circulating iron within macrophages, thus reducing serum concentrations. This occurs more rapidly than generation of fibrinogen and is significant by 16 hours of the inflammatory stimulus. Possible limitations of iron are that it may not be as reliable as an inflammatory marker in young foals
and it may also increase in response to general anaesthesia.

SAA responds more rapidly than other acute phase proteins (within 8-12 hours) and may therefore be more helpful in assessing acute inflammatory disease. Furthermore, most normal horses have SAA concentrations of zero and with severe inflammatory disorders this
can rise markedly. Compared with fibrinogen this allows for better ‘grading’ of severity of the inflammatory process and more sensitive monitoring of progress. Because of the steep time-curve of SAA there can be marked changes on a day-to-day basis and continued increases indicate an active inflammatory process and recommendation to continue treatment, or at least monitoring. Unfortunately, the “trade-off” with simpler horse-side SAA analysers is that results generated are inconsistent and less able to accurately detect changes (up or down) as the case progresses and apparent trends in their results must be treated cautiously.

Globulins are often seen to increase in inflammatory disease as most of the acute phase proteins are included in the globulin fraction. The profile of globulins tends to follow a more chronic time course and is slow to normalise following recovery from inflammation. In addition to being an indicator of chronic inflammation, hyperglobulinaemia is seen commonly with hepatopathy. Therefore, liver enzymes should always be cross checked when high globulins are found. There is no evidence of any benefit of sub-classifying globulins further using serum protein electrophoresis as this technique very rarely provides further useful diagnostic or prognostic information unless a severe monoclonal hyperglobulinaemia is seen which could be secondary to a rare neoplasm such as myeloma. Contrary to popular myth, serum protein electrophoresis cannot differentiate causes of high globulin such as bacterial infection, parasitism, liver disease etc…

Serum albumin is often referred to as a “negative acute phase protein” as albumin levels can
fall slightly with inflammation (especially in chronic cases) as amino acids are utilized for
synthesis of acute phase proteins. Hypoalbuminaemia as a result of inflammation tends to be
relatively mild and very rarely lower than 25 g/L. Hepatopathy is also a potential but rare
cause of mild hypoalbuminaemia. Marked hypoalbuminaemia (<20-25 g/L) is generally
indicative of loss of albumin rather than reduced synthesis and the most likely causes are
protein-losing enteropathy (e.g. parasites, NSAID use, IBD etc..) or loss into an effusion.
Occasional cases of protein losing nephropathy as a result of glomerular disease are also seen
and tests for renal injury are indicated if there are no signs of enteric disease (creatinine, urea,
SDMA, calcium).