Electrosurgery, patient motion and some types of lighting can cause errors in saturation readout; it is recommended that probes should be shielded from ambient lighting. Intravenous dyes can introduce gross but transient errors, which may also be present in in vitro measurements. Carboxyhaemoglobin causes overestimation of fractional saturation by an amount less than, but possibly close to, the percent of carboxyhaemoglobin present. Methaemoglobin causes the pulse oximeter readout to tend towards 85%. Fetal haemoglobin and bilirubin introduce no significant error, although they may interfere with in vitro measurements. Skin pigmentation can result in a slight decrease in accuracy. Nail polish may cause up to 6% underestimation of saturation; it is recommended that probes should be mounted sideways on fingers with nail polish or long nails. Adhesive tape or a vinyl glove across the probe has no demonstrable effect on accuracy. A blood sample should be analysed by a multiwavelength in vitro oximeter when an erroneous pulse oximeter reading is suspected, although errors may be introduced in the in vitro reading by fetal haemoglobin, bilirubin and intravenous dyes.