Metformin and contrast dye for scans: kidney checks and what to do

Metformin is a tablet used to treat type 2 diabetes. The NHS says it may not be suitable for people with liver and kidney problems, and that taking an extra dose can be dangerous.

Contrast dye is a different matter. The NHS CT page explains that, depending on why you are having a CT scan, you may be given a dye called contrast medium to show more detail. The NHS acute kidney injury page lists the dye sometimes injected as part of scans such as a CT scan or angiography as one possible cause of acute kidney injury.

The link between the two is therefore the kidneys. A Leeds Teaching Hospitals leaflet describes its precautions as protecting patients from any risk of interaction between metformin tablets and the contrast agent, and from the effects that it may have on the kidneys.

The NHS metformin page lists lactic acidosis, too much acid in the blood, as a rare but serious side effect, with symptoms including being sick, stomach pain, muscle cramps, breathing problems and feeling severely tired, and says to call 999 if you think you might be experiencing it. The BNF notes that excessive alcohol consumption potentially increases the risk of lactic acidosis with metformin.

Acute kidney injury is the other end of the problem. The NHS describes it as a sudden problem that stops the kidneys working as well as they should, which can be life-threatening if not treated, although many people recover fully within a few weeks. Pausing metformin for a short period is a precaution to avoid adding to that picture.

There is no single national patient instruction, so the detail comes from your hospital. A Leeds Teaching Hospitals patient leaflet for people having a CT scan with contrast whose kidney function is not within normal limits states that you should not take metformin tablets for 48 hours following the injection, that stopping metformin is unlikely to make a significant difference to diabetes control, and to be aware of any signs that your blood sugar is off balance.

Other hospitals publish their own policies, so the detail can differ. Your appointment letter or the radiology department can tell you what applies to you, including whether a recent kidney blood test is needed.

• Bring a list of your medicines, including the dose of metformin.

• Mention diabetes and any kidney problems when you book and again at the scan, as the NHS asks.

• Ask whether a recent kidney blood test is needed and whether you should pause metformin.

• If told to pause, ask exactly when to restart, and whether a blood test is needed first.

Follow the instruction you are given and do not extend or shorten it. The Leeds leaflet advises being aware of signs that blood sugar is off balance, and contacting the diabetes nurse or doctor who normally treats your diabetes if you have concerns. If you check your blood glucose at home, check as you usually would, and record the readings.

Other diabetes medicines are a separate matter. If you take insulin or another tablet, ask the team whether any change applies, since the advice for metformin does not automatically cover them.

After a contrast scan the NHS says some people can have an allergic reaction to the dye, and that you may be asked to wait at the hospital for up to 30 minutes afterwards. If you feel unwell later, with peeing much less than usual, swollen feet or legs, nausea or vomiting, or unusual tiredness or breathlessness, the NHS AKI page advises asking for an urgent GP appointment or getting help from NHS 111.

The NHS metformin page says to call 999 if you think you are having lactic acidosis or anaphylaxis, and NHS 111 for other serious side effects. Persistent vomiting while you are unwell can itself lead to dehydration, which the NHS metformin page flags as a risk if you are ill with vomiting, diarrhoea or high temperature.

The Leeds leaflet frames the pause as a short precaution after the injection, with metformin restarting as advised. If you are in doubt about any instruction, telephoning the department before the scan is better than guessing.

This is the same whether the scan is an NHS or private appointment, since the same safety principles apply; only the local protocol differs.