Background: Up to a 20-fold variation in serum cardiac troponin I (cTnI) co
ncentration may be observed for a given patient sample with different analy
tical methods. Because more limited variation is seen for control materials
and for purified cTnI, we explored the possibility that cTnI was present i
n altered forms in serum.
Methods: We used four recombinantly engineered cTnI fragments to study the
regions of cTnI recognized by the Stratus(R), Opus(R), and ACCESS(R) immuno
assays. The stability of these regions in serum was analyzed with Western b
lot.
Results: The measurement of several control materials and different forms o
f purified cTnI using selected commercial assays demonstrated five- to nine
fold variation. Both the Stratus and Opus assays recognized the N-terminal
portion (NTP) of cTnI, whereas the ACCESS assay recognized the C-terminal p
ortion (CTP) of cTnI. Incubation of recombinant cTnI in normal human serum
produced a marked decrease in cTnI concentration as determined with the ACC
ESS, but not the Stratus, immunoassay. Western blot analysis of the same sa
mples using cTnI NTP- and CTP-specific antibodies demonstrated preferential
degradation of the CTP of cTnI.
Conclusions: The availability of serum cTnI epitopes is markedly affected b
y the extent of ligand degradation. The N-terminal half of the cTnI molecul
e was found to be the most stable region in human serum. Differential degra
dation of cTnI is a key factor in assay-to-assay variation. (C) 1999 Americ
an Association for Clinical Chemistry.