The dynamic nature of SAA makes it an ideal marker for monitoring progression or resolution of illness, including response to treatment.1-6 It is rapidly responsive to clinical changes and directly reflects the inflammatory status of the horse. Point-of-care testing with numerical results that can be compared and tracked over time facilitates this process.
SAA may continue to increase for up to 4 days following an acute inflammatory insult, even in the face of treatment (Fig. 1). It is necessary to consider this when monitoring treatment response. Depending on the timing of initial examination, SAA may not yet have peaked when the horse is evaluated. This could result in a similar or possibly higher value being obtained later if follow up testing is not planned carefully. If SAA does not decrease appropriately, it likely indicates that the chosen treatment is not efficacious, and alternatives should be considered.6
Ideally, patients should be tested during the initial exam and then be retested in 24-48 hours in case SAA increases further (Fig. 1). Identifying the peak value will help with interpretation of subsequent results. Different peak values could indicate different disease processes, and higher values will take more time to return to normal. To evaluate efficacy of antimicrobials or other treatments, follow up testing should be performed 3-4 days after the start of treatment, or 1-2 days after peak [SAA] has been identified. If treatment is effective and inflammation/infection is resolving, SAA should begin to decline (Fig. 1).6 Follow up assessments can be performed every 24-48 hours if desired to document continued clinical improvement. If at any time the SAA stops decreasing or starts to increase, treatment failure or additional complications should be considered.6 At a minimum, it is advisable to test SAA before discontinuing therapy and/or prior to discharge (if hospitalized) to verify that the horse has returned to normal.
Monitoring is also useful for early identification of complications post-operatively or with significant medical conditions.1-5,7-11 Surgery itself will also cause an increase but understanding the pattern and degree of this increase allows SAA to still be used as a very effective post-operative monitoring tool. For post-operative monitoring, SAA can be re-evaluated every 24-48 hours, depending on budget and patient proximity. SAA should peak at 2-4 days post-op (depending on procedure) and then gradually decline (Fig. 1).7,12 If post-operative values remain high, or spike again later during recovery, complications are highly likely. 3,7-11 As early identification can be difficult, this type of monitoring can be invaluable, providing a trigger for more in-depth exploration of possible causes.
It is important to remember that SAA measured at one specific time does not necessarily reflect the overall peak as animals may be transitioning from local to systemic inflammation or between chronic and acute states.6 This underscores the particular value of SAA for monitoring patients. It mirrors clinical condition much more closely than other diagnostic tools such as WBC count or fibrinogen1,13,14, and unlike body temperature should not be affected by NSAID therapy.13 SAA does not answer all clinical questions but adds immediate objective input to help guide clinical decision-making.
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14 Hultén C, Demmers S. Serum amyloid A (SAA) as an aid in the management of infectious disease in the foal: comparison with total leucocyte count, neutrophil count and fibrinogen. Equine Vet J 2002;34(7):693-698.