What is the reason for removing make up and nail varnish before surgery?

When having surgery, the medical team will want to monitor your oxygen levels in your blood (usually in a non invasive way). This will be done by attaching a pulse oximetry machine to your finger or toe.

The machine is trying to measure how much haemoglobin is in your system. If you're wearing nail polish, it creates a barrier and an inaccurate reading.