The signal-to-cutoff ratio for an enzyme immunoassay (EIA) result in anti-Hepatitis C antibody testing can help distinguish low risk from high risk patients for hepatitis C viral infections.
cutoff point for absorbance =
= (mean for all negative values) + 0.6
A patient test is reported as positive if it is greater than the cutoff value. If a test is positive, then a second repeat test is performed. If the second test is positive then the result is reported as “repeatedly positive.”
signal-to-cutoff ratio =
= (signal for patient in absorbance units) / (cutoff absorbance level)
Interpretation:
• A signal-to-cutoff ratio < 3.8 is reported as low and a ratio >= 3.8 is reported as high.
• A low signal-to-cutoff ratio often is a false positive.
• A high signal-to-cutoff ratio is often a true positive.
S/C Ratio
Percent HCV RNA Positive
1.0 – 1.5
7%
1.6 – 3.5
12 – 17%
3.6 – 3.7
20%
>= 3.8
87 – 91%
from Table 3, page 482, Dufour et al
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