Table 3– Results of the diagnostic procedure according to a theoretical approach based primarily on quantitative PCR Pneumocystis jirovecii as the first test^#

First test	Second test	Results
PCR P. jirovecii copies·mL⁻¹	IF	Total subjects	Definitive PCP (IF test positive)	Possible PCP	PCP excluded
>1450	Positive	50 (98.0)	50 (100)	0	0
	Negative	1 (2.0)	0	0	1 (100)
85–1450	Positive	21 (70)	21 (100)	0	0
	Negative	9 (30)	0	2 (22.2)	7 (77.7)
1–85	Positive	0	0
	Negative	8 (100)	0	1 (12.5)	7 (87.5)
Negative	Positive	0	0
	Negative	153 (100)	0	1 (0.65)^¶	152 (99.3)

Data are presented as n (%). IF: immunofluorescence; PCP: Pneumocystitis pneumonia. ^#: definitive conclusions are possible only with high copy numbers (>1,450 copies·mL⁻¹) and negative results. In the range 1–1,450 copies·mL⁻¹, quantitative PCR is not able to differentiate between “possible PCP” and “PCP excluded” and clinical suspicion remains essential. ^¶: PCP could not be formally ruled out in only one patient with rheumatoid arthritis and lung involvement. The treating physicians prescribed 3-week therapy against Pneumocystis jirovecii, but judged the acute lung disease as “more likely to be caused by autoimmune disease exacerbation rather than PCP” in the discharge letter. Retrospective reviewers unaware of the PCR result agreed with this interpretation.