However, many of the stimulated tests were carried out in the lab for a relatively short period of time.
Histopathological examination of corresponding cochlear sections comparing the stimulated test side with the unstimulated control side showed no significant difference (P>0.05) for either animals implanted with HiQ electrodes (n=6) or standard electrodes (n=2).
Nor were there any significant differences between the spiral ganglion cell density of the basal turn implanted with HiQ or standard electrodes for both the stimulated test (P=0.31) and the unstimulated control side (P=0.84).
Values close to clinical thresholds could be repeated or a more rigorous stimulated test (mixed-meal or glucagon tests) performed.
Any stimulated test is expensive and time consuming and thus may not provide benefits in clinical practice beyond a fasting or a random C-peptide measurement.
For the stimulated test, 1 μL (535 μmol/L) of PMA (Phorbol 12-myristate 13-acetate, Cat. P8139, Sigma) was added.
The spread of C-peptide levels is wide and a high or very low level will exclude or confirm severe insulin deficiency; a fasted or stimulated test could then be conducted should the result be indeterminate.
A cut-off value of ≤ 80 min5 min for the stimulated test showed a sensitivity of 87.6%, specificity of 67.5%, positive predictive value of 55.7% and a negative predictive value of 7.8%.
Without a formal study using a fasting or random C-peptide value in addition to a stimulated test, it is not possible to determine which is more clinically meaningful in an individual subject.
Cut-off values for the unstimulated test (≤ 30 min5 min, ≤ 40 min5 min, ≤ 50 min5 min, ≤ 60 min5 min), and for the stimulated test (≤ 50 min5 min, ≤ 60 min5 min, ≤ 70 min5 min, ≤ 80 min5 min) were examined and sensitivity, specificity, positive predictive values and negative predictive values were calculated.
