Aim: to study the clinical significance and potential utility of measuring serum amyloid A protein (SAA) compared with the classical acute phase protein, C-reactive protein (CRP).
Method: a 3 month prospective study on 66 women, mean age 83 years (range 69-106) and 33 men, mean age 84 years (range 69-95), admitted to the geriatric medicine unit at Hammersmith Hospital. CRP and SAA were determined on admission and at intervals throughout hospital stay; outcome end-points were death during the study, detection of infection, duration of admission and early re-admission to hospital after discharge.
Results: CRP and SAA responses were highly correlated (r = 0.75, P = 0.0001). However, the SAA response was greater than that of CRP in most individuals, with a median ratio of initial SAA to CRP of 2.2 in patients with infective pathology and 1.6 in those with inflammatory pathology. Median (range) SAA on admission was 98 (0.1-940) mg/ml in patients with infection and was twice that observed in patients with other causes of inflammation, median value 50 (0.6-699) mg/l. There was no difference between median CRP on admission in patients with infection or inflammation, median value 53 (0.1-235) and 51.5 (5-246) mg/l respectively. Initial and peak levels of CRP, but not of SAA, were significantly greater in patients who subsequently died, whereas high levels of both proteins predicted length of admission and early re-admission.
Conclusion: major elevations of the serum concentrations of CRP and SAA indicated serious disease and predicted poor outcome. Measurement of SAA as well as CRP enhanced the clinical utility of monitoring the acute phase response in 7% of patients with a diagnosis of infection.