Figure 3.
Influence of internal control cycle-threshold (Ct) on calculated Spearman rank. The calculated Spearman rank correlation (rs) between Mycobacterium tuberculosis Ct and acid-fast bacilli smear grade for all concentrated sputum samples is shown for each maximum allowed internal control Ct. The arrow indicates the point of maximum correlation. The dotted line indicates the trend in Spearman rank correlation that would have been expected in the absence of polymerase chain reaction inhibition.