FIG. 2.
(A) Correlation of CT values with viral RNA copies among fatal (n = 18) and nonfatal (n = 36) cases of RVF. The RVF RNA concentration in serum was calculated using a standard curve developed using a qRT-PCR as described in Materials and Methods. The ≥95% chance limit of detection as determined by probit regression analysis by Drosten et al. (7) was 2,835 RNA copies/ml (95% confidence interval, 2,143 to 4,525) of serum. The CT values were determined from a one-step real-time RT-PCR, with fluorescence read at the combined annealing-extension step at 57°C. Using a cutoff CT value of 27.0, cases registering CT values of ≤27.0 were associated with a CFR of ≥50.0, whereas those with CT values of >27.0 had a CFR of 4.5%, a sensitivity of 93.8%, and a negative predictive value of 95.5%. (B) Correlation of levels of infectious RVF virus in serum (viremia), with CT values among fatal (n = 18) and nonfatal (n = 36) cases of RVF. Viremia was determined by inoculating patient sera in Vero cells and expressed as TCID50/ml of serum. The limit of detection was 100 infectious RVF virus particles per ml of serum. The mean infectious virus levels were fourfold higher in fatal cases than those in nonfatal cases of RVF. No infectious virus was detected in 20 of the 36 nonfatal cases, all of which had CT values of >27.0, whereas infectious virus titers ranging from 101.3 to 107.8 TCID50 were detected in all the fatal cases.