Periprosthetic infection: where do we stand with regard to Gram stain?

Figures & data

Table 1. The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of the 2 diagnostic arms of Gram staining

Diagnostic arm	Sensitivity (%)	Specificity (%)	PPV (%)	NPV (%)
THA
Presence of organisms	31 (23–38)	100 (99–100)	100 (92–100)	79 (76–83)
Presence of “many” PMNs	31 (24–39)	99 (97–99)	89 (80–97)	79 (75–83)
TKA
Presence of organisms	30 (24–37)	100 (99–100)	98 (94–100)	70 (66–75)
Presence of “many” PMNs	53 (46–61)	99 (97–99)	96 (92–99)	78 (73–82)

Table 2. Combinations of the 2 arms of Gram staining were performed in series and in parallel for both types of joints. PPV: positive predictive value; NPV: negative predictive value

Combinations	Sensitivity (%)	Specificity (%)	PPV (%)	NPV (%)
THA
Series	18 (12–24)	100 (99–100)	100 (87–100)	77 (73–80)
Parallel	43 (35–51)	99 (97–99)	93 (84–98)	82 (79–86)
TKA
Series	21 (15–27)	100 (99–100)	100 (90–100)	68 (63–72)
Parallel	64 (57–72)	99 (97–99)	97 (91–99)	82 (77–86)

Table 3. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for increasing specimen numbers obtained intraoperatively

Sample No. Test	Sensitivity (%)	Specificity (%)	PPV (%)	NPV (%)
1
Presence of organisms	32 (20–44)	100 (99–100)	95 (85–100)	89 (86–92)
Presence of “many” PMNs	29 (17–40)	99 (98–99)	90 (76–100)	89 (85–92)
2
Presence of organisms	28 (20–35)	100 (98–100)	100 (91–100)	72 (67–76)
Presence of “many” PMNs	43 (35–51)	97 (95–99)	90 (82–97)	76 (75–80)
3
Presence of organisms	29 (20–40)	100 (95–100)	100 (86–100)	55 (46–63)
Presence of “many” PMNs	47 (36–58)	97 (93–100)	95 (89–100)	61 (51–70)
4
Presence of organisms	31 (50–84)	100 (80–100)	100 (69–100)	44 (28–61)
Presence of “many” PMNs	50 (33–67)	100 (80–100)	100 (79–100)	52 (34–68)