Table 3. Evidence summary: Do closed suction wound drains decrease the incidence of postoperative complications?

Outcome	Overall quality of evidence	Studies	Effect size
			Range of upper bound of 95% confidence intervala
Hematoma	Low due to risk of bias and imprecision	3 RCTs (n = 333)	Drain: 2.9–13.6% No drain: 3.1–16.7%
		2 retro cohorts (n = 962)	Drain: 1.1–1.6% No drain: 1.1–2.6%
Superficial wound infection	Insufficient due to risk of bias and imprecision	2 RCTs (n = 283)	Drain: 4.6–7.1% No drain: 3.0–7.3%
		2 retro cohorts (n = 962)	Drain: 1.0–5.7% No drain: 1.1–5.4%
Deep infection	Insufficient due to risk of bias and imprecision	1 RCT (n = 83)	Drain: 7.1% No drain: 7.3%
		1 retro cohort (n = 560)	Drain: 1.0% No drain: 1.1%
Postoperative blood transfusion	Insufficient due to risk of bias	1 retro cohort (n = 402)	Drain: 28.8% No drain: 11.4% RR: 3.5 (1.7, 7.0)

Abbreviations: RCT, randomized controlled trial; retro, retrospective cohort; RR, risk ratio.

^aCalculated using Hanley's rule of three when zero events are reported for a given outcome.