Contrast-induced nephropathy is an important cause of acute renal failure and is associated with increased morbidity and mortality in hospitalized patients.15,42
Our analysis suggests that administration of NAC around the time of contrast administration prevents renal injury. Patients treated with NAC had both a lower mean creatinine (difference in mean ΔCr=−0.27 mg/dl; 95% CI, −0.43 to −0.11) and a reduced risk of developing CIN compared with control patients (RR, 0.43; 95% CI, 0.24 to 0.75). In our sensitivity analyses, use of NAC was protective against renal injury in almost all subgroups that included at least 3 studies.
A particular strength of our study was our analysis of two separate outcome measures, both showing a benefit from NAC administration. Our use of a second outcome measure was particularly important given the varying definitions of CIN used in the studies. Some studies reported CIN as a rise in creatinine of 0.5 mg/dl, while others reported CIN as a 25% increase in serum creatinine. Differing definitions of CIN make comparison between studies more difficult. Having change in creatinine as a second outcome measure allowed us to validate the findings of our first outcome measure, and provided an outcome measure that may be easier to interpret for most clinicians.
Despite the consistent results between the two outcome measures, the summary estimates must be interpreted with caution due to the presence of heterogeneity. We attempted to identify sources of heterogeneity by examining subgroups of studies with similar features. However, heterogeneity was present in almost all subgroups regardless of study characteristics including prevalence of diabetes, high baseline creatinine, high- or low-contrast dose, and high and low study quality. In particular, sensitivity analysis eliminating the study using intravenous NAC29
or the study using intravenous rather than intra-arterial contrast28
failed to reduce the heterogeneity of the studies. The persistent heterogeneity suggests that there are differences in patient populations or study methodology that we were unable to identify. We were not able to perform sensitivity analysis on the different formulations of NAC as the specific oral formulation of NAC was not consistently reported in the studies.
Control of independent risk factors for renal dysfunction was not noted in most studies. For example, many of the medications that could affect renal function are not described clearly in the included studies. Medications such as furosemide may impair renal function in the setting of contrast administration.43
Other medications such as nonsteroidal anti-inflammatory drugs and angiotensin-converting enzyme inhibitor (ACEi) may affect renal function or the efficacy of NAC itself. Without more information, it is not possible to determine the contribution of these medications to the observed outcome measures.
Many of the studies did not receive high-quality scores. However, most of these studies did not specify whether or not they fit the quality criteria. The studies, therefore, could have had higher-quality scores than realized.
N-acetylcysteine is inexpensive, easy to administer, and appears to have no significant toxicity. Our analysis suggests that it is effective for preventing the increase in serum creatinine caused by contrast administration, especially in the setting of cardiac catheterization. While the change in serum creatinine was only assessed at 48 hours, prior studies have documented that large changes in serum creatinine (CIN, or a change of >0.5 mg/dl) are associated with in-hospital morbidity and mortality.1,5–9
While the incidence is rare, it is also possible in those with advanced underlying renal disease that CIN could lead to a permanent loss of renal function and therefore earlier dialysis.44
Given the apparent low cost of NAC and the potential benefits, one might reasonably argue that NAC should be recommended for widespread use prior to contrast administration in all patients.
However, a major limitation of all of the included studies is the focus on the short-term changes in creatinine rather than on longer-term and more clinically relevant outcomes such as the incidence of dialysis and other adverse events during hospitalization, the length of hospitalization, and the rate of progression to end-stage renal disease after discharge. Only 1 study reported information on length of hospitalization and oliguria, and none of the studies reported the effect of NAC on overall costs or outcomes after discharge. Of the 7 studies that reported the incidence of dialysis among participants, dialysis was required in only 2 patients in the control group and in none in the NAC group. Although our meta-analysis had limited power to show a benefit from NAC administration in terms of preventing dialysis (due to the small number of patients requiring dialysis), our study did show that this outcome is rare (0.2%). Therefore, when emergent intravenous contrast is indicated, our data suggest that there should not be a delay in diagnostic or therapeutic procedures for administration of NAC. A related concern is that the widespread adoption of NAC may delay the identification and implementation of other potentially more beneficial interventions to reduce CIN.
To address the lack of information regarding clinically relevant outcomes, we believe that randomized controlled trials are needed to assess the effect of NAC on important long-term clinical outcomes and overall costs. Ideally, these studies should use a standardized dose and schedule of NAC, fluid hydration, contrast, and placebo, and should include outcomes such as renal failure, rates of dialysis, death, major cardiac events, and length of hospitalization. In addition, the baseline populations should be clearly defined in order to determine the effect of baseline renal function, diabetes, and concomitant medication use on outcomes.
A recently published systematic review concluded that NAC reduces the risk of CIN by 56% compared with control patients, but this study did not examine continuous outcomes (change in creatinine) or the incidence of dialysis among study patients. Furthermore, the results were limited by the finding of publication bias. In our study, we found no evidence of publication bias when examining our main outcome measure, the difference in mean change in creatinine between treatment groups. When assessing the presence of publication bias in the secondary outcome measure of CIN, we found publication bias only when using the same outcome measure as that used in the earlier study (i.e., relative risk; ). This variability of results in tests of publication bias based on methodology has been described previously in the literature.45,46
While there is no evidence that one methodology of assessing publication bias is superior another, interpretation of publication bias based on risk differences would be in agreement with the results of our primary outcome measure. In our primary outcome measure, publication bias is not present and does not affect the accuracy of the summary estimate of the effect of NAC for the prevention of CIN.
Our study also identified 6 additional studies2,23,29–31,41
not mentioned in the prior review. Two of these studies satisfied all inclusion criteria and were included in our meta-analysis.23,29
We also performed a more extensive sensitivity analysis to examine the effect of patient characteristics and study design on heterogeneity, but were unable to identify the source of heterogeneity. However, the identification of 2 additional studies provided a more precise estimate of the effect of NAC in the subgroup of patients undergoing cardiac catheterization. We found a statistically significant benefit from NAC administration in these patients, while the prior study did not.47
There remain several important limitations to our meta-analysis, including the fact that many of the studies did not receive high-quality scores. Most of these studies did not specify whether or not they met the quality criteria, and the true quality of the studies therefore remains uncertain. Another important limitation is the inconsistent reporting among included studies of baseline patient characteristics and methods. In particular, lack of information regarding patient characteristics makes it difficult to identify subgroups that may or may not benefit from NAC. Furthermore, lack of more detailed information on the formulation of NAC used made it difficult to determine the optimal method of delivering the medication. Despite this limitation, we were able to use available data to identify a subgroup of patients undergoing cardiac catheterization that may benefit from NAC administration.
Finally, our ability to translate a reduced incidence of CIN and a lower mean creatinine to applicable clinical outcomes such as length of hospitalization and progression to end-stage renal disease is limited by the available data. In our analysis, the incidence of dialysis was rare, occurring in only 0.2% of patients. Further studies are needed to assess the impact of NAC on these useful clinical endpoints.
Our meta-analysis suggests that administration of NAC around the time of contrast delivery protects against worsening renal function and CIN at 48 hours. However, the incidence of dialysis is rare, and the long-term effect of this agent on more clinically important outcomes is not established. Widespread use of this agent has the potential to delay diagnostic or therapeutic interventions. Further studies are needed to assess the long-term effects of this agent and the overall cost-effectiveness of routine NAC administration prior to contrast delivery, particularly in high-risk patients.