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Table 4 Relative and absolute incremental yields and NND for smear and cultures

From: High yield of culture-based diagnosis in a TB-endemic setting

  Relative incremental yield vs.
Yield 1st S 2 S total 1st C
Expected # of P after 1 S = 377/2537 = 14.9%    
Expected # of P after 2 S = 508/2537 = 20.0% 1.3   
Expected # of P after 1 C = 682/2537 = 26.9% 1.8 1.3  
Expected # of P after 2 C = 760/2537 = 29.9% 2.0 1.5 1.1
  Absolute incremental yield vs.
Yield 1st S 2 S total 1st C
Expected # of P after 1 S = 377/2537 = 14.9%    
Expected # of P after 2 S = 508/2537 = 20.0% 5.2%   
Expected # of P after 1 C = 682/2537 = 26.9% 12.0% 6.9%  
Expected # of P after 2 C = 760/2537 = 29.9% 15.1% 9.9% 3.1%
  NND vs.   
  NND 1st S 2 S total 1st C
1st Smear 7    
2 Smears total 5 19   
1st Culture 4 8 15  
2 Cultures total 3 7 10 33
  1. Legend: S = smear, P = positive, N = negative, SE = Suspect Episodes, NND = number needed to diagnose (see Methods for details), IY = incremental yield. Note: Expected # of P after 1 S or C is same as Observed (see Appendix for details).