Because of the increasing number of patients added to the transplant waiting list and the lack of sufficient standard criteria donors (SCD) (brain death donors less than 50 years old) to meet this demand, DCD kidneys are now being accepted by many organ transplant centers all over the world as a potential organ source 
. Previous clinical studies showed that this kind of kidney had a higher risk of complications such as DGF when compared to SCD kidneys. Some studies demonstrated that MP preservation could reduce DGF rate in both SCD and DCD kidneys. In 2003, a meta-analysis showed that MP led to a 20% reduction in the incidence of DGF 
. However, there was considerable clinical heterogeneity among the involved studies, which included prospective RCTs with small sample size and even retrospective non-RCTs. This conclusion was thought to be premature to advocate the widespread use of MP into clinical kidney transplantation 
. Thereafter, two multicenter RCTs were conducted in Europe and controversial conclusions were drawn. Our analysis, based on prospective RCTs of DCD kidneys, identified that MP is superior to CS in lowering the incidence of DGF in DCD kidneys, with a ~40% reduction in DGF when organs are preserved by MP compared to those preserved by CS. We found no prospective RCT study conducted in the US using our searching strategy. In 2006, data from Scientific Registry of Transplant Recipients (SRTR) concerning the use of DCD organs showed that the rates of DGF were not significantly different between MP and CS for DCD kidneys (40.2% vs. 42.3%, P
0.15) (data from 2000 to 2004) 
. A meeting abstract using the OPTN/UNOS database (from 2004 to 2009) also found that there was no difference in DGF rate between MP and CS (43.1% vs. 42.3%, P
0.83) for DCD kidneys. However, another publication based on OPTN data (from 1993 to November 2008) analyzing DCD donors found that MP did reduce DGF incidence compared with CS when donor age was >60 years and improved long term graft survival when donor age was >50 years 
. Importantly, the ratio of controlled to uncontrolled DCD is higher in the US than in Europe, and the incidence of DGF is lower in the US than in Europe (~40% vs. ~60%) 
. Overall, although no subgroup analysis was done in the present study, we believe MP benefits early outcome (DGF) of DCD kidneys, especially when the organs are from expanded-criteria donors (ECD) (brain death donors more than 60 years old or between 50 and 59 years of age with 2 of the 3 additional risk factors) and uncontrolled DCD donors.
We found no difference in the incidence of PNF between the two preservation methods in the current analysis. This may be true or may be due to an inability to detect an effect of MP because of the relatively low incidence of PNF (~5%) after DCD kidney transplant. One would expect that the reduction of DGF by machine perfusion might lead to an increase in graft and patient survivals since previous studies suggested that DGF was associated with reduced graft survival, which was observed in DBD kidneys. However, in our study, the one year graft and patient survival rates were similar between MP and CS preserved DCD kidneys. One US study 
also revealed the same results based on a retrospective cohort of the OPTN/UNOS database (from 2004 to 2009) (The mean follow-up time was 2.2±2.6 years with a range of 0–15 years). The different nature of DGF between DCD and DBD kidneys may be one of the underlying reasons for this observation. Metabolic, hemodynamic, hormonal, and inflammatory changes triggered by brain death 
, but not cardiac death may impair kidney function more than warm ischemic injury alone, thus affecting long-term outcomes of DBD kidneys more 
. Though MP was not shown to benefit one year graft and patient survivals based on the current study, it may increase long-term graft and patient survivals because of the benefit of reducing DGF incidence since DGF has been correlated with decreased long-term graft survival 
. Indeed, one recent multicenter RCT found that the 3-year DBD kidneys allograft survival preserved by MP is better than those with CS preservation, especially in ECD kidneys 
Notably, MP needs additional logistic requirements and costs in comparison with conventional CS preservation before the kidneys are implanted. However, reduced requirement for dialysis after transplantation due to a lower rate of DGF and a shorter duration of DGF after transplantation could compensate for these additional logistic requirement costs by MP. No included study assessed the cost-effectiveness of MP and CS for DD kidneys in this analysis, but one multicenter RCT study 
evaluated the short-term cost-effectiveness and showed that total initial hospitalization cost is less for MP than for CS, suggesting that MP can lower the costs per life-year and reduce costs per quality-adjusted life-year (QALY) when compared to CS.
There are some limitations in the present study. Firstly, clinical heterogeneity between studies might exist although we had strict enrollment criteria of references (only included prospective RCTs). One of the studies 
used the cryoprecipitated plasma perfusate for MP while the two most recent studies used the kidney preservation solution-1 (KPS-1) 
. In addition, the current analysis only presented data about the short term patient and graft survival, MP likely affect the long term patient and graft survival. Furthermore, this study included all four kinds of DCD donors (Maastricht classification: I: Brought in dead; II: Unsuccessful resuscitation; III: Awaiting cardiac arrest in hospital; IV: Cardiac arrest after brain-stem death) though category III DCD donor was the main portion of the study population and the only type of subject in the two multicenter RCTs 
. Thus, both controlled and uncontrolled cardiac death donors were pooled together in our analysis. This may result in insufficient evidence to determine whether this result is suitable for controlled or uncontrolled DCD donors only. Additionally, though we included studies no more than 20 years old, clinical practice of MP during this period may differ between studies as stated in the Results
section. Finally, none of the included studies performed a cost-effectiveness analysis. These limitations provide room for future studies.
In conclusion, MP preservation of kidneys obtained from DCD donors can reduce DGF incidence after transplantation in comparison to conventional CS preservation. However, PNF incidence and one year graft and patient survivals were not different in patients using the two technologies. MP preservation is recommended for DCD kidneys to minimize the risk of DGF.