Radiographic contrast media and metformin

Editor, – I write regarding the article dealing with radiographic contrast media (Aust Prescr 2010;33:19-22).

I have recently authored a systematic review relating to the safety of iodinated contrast in patients receiving metformin.¹ The review found no evidence to substantiate beliefs about the need to cease metformin in individuals with stable, normal renal function who were to have a 'normal' amount of intravenous iodinated contrast for an examination such as a CT scan. Despite a number of international guidelines having disparate recommendations about cessation of metformin, the Royal Australian and New Zealand College of Radiologists (RANZCR), the Royal College of Radiologists (RCR) and the European Society of Urogenital Radiology guidelines recommend that there is no need to stop metformin in these patients. The RANZCR recommendations are based on the extremely low risk of precipitation of contrast-induced nephropathy in this group. The Australian and RCR guidelines were modified along these lines in March and June 2009, respectively, soon after the systematic review was presented at the Radiological Society of North America meeting in December 2008.

Other work by Jeffrey Newhouse supports our findings that the risk of contrast-induced nephropathy has been exaggerated by research focusing on patients who have large volume, intra-arterial administration of iodinated media and by the lack of a genuine control group in many of the studies that have linked iodinated media to high rates of post-procedural contrast-induced nephropathy.

The advice by the radiologist to cease metformin, when this is not necessary, can have many unintended consequences such as the patient forgetting to recommence metformin. In addition, patients may visit their general practitioner for advice about when it is safe to recommence metformin, incurring costs to the health system.

The advice given in the Australian Prescriber article is entirely appropriate for patients who:

• are having large contrast volume, intra-arterial procedures (such as coronary angiography or interventional procedures) or
• are known to have abnormal or acutely deteriorating renal function.

However, this important distinction is not made clear in the article and general practitioners may interpret this advice to apply to their own practice context, which is largely CT scanning or other lower dose procedures associated with intravenous contrast media.

Stacy Goergen
Associate Professor, Director of Research
Department of Diagnostic Imaging
Southern Health Clayton, Vic.

Professor Ken Thompson and Dr Dinesh Varma, authors of the article, comment:

When writing this article we were well aware of the RANZCR guidelines and the issues of how to handle a patient with type 2 diabetes taking metformin who requires a contrast CT.

The RANZCR guidelines agree that it is difficult to measure estimated glomerular filtration rate (e-GFR) in all patients in an outpatient setting, although this is our practice.

While it is true that there is little or no high level evidence to recommend stopping metformin in patients with normal, stable renal function receiving a moderate dose of contrast media, the general practitioner who requests the contrast examination has no control over the actual amount of contrast media the patient is given.This may vary for a wide variety of reasons. An extremely low risk is not the same as no risk.

We were also influenced by the drug manufacturer's information and decided to provide advice that is consistent with the packaging information. In our view, the risk that a patient who takes a drug every day will forget to recommence the drug is unlikely.