Mistake Reading Cat Microchip
It is possible for a cat to end up being mistakenly re-homed even when microchipped. The microchip is the best identification method we have but it is not foolproof. An example proves the point, although in the story below the cat’s owner made some poor decisions which exacerbated the situation and enhanced the chance of losing her cat despite being microchipped.

Microchips are not infallible. Human error is probably the weak link.
A woman, Sam Cotton, unfortunately lost her home, at least temporary, due to a bad fire. She had to vacate and live in temporary accommodation. She wrongly decided to let her cat, Tiggy, live in the burnt out ruins of her home whilst she visited daily to feed her. Eventually her cat disappeared which does not surprise me, to be honest.
The home had changed out of all recognition which would have meant Tiggy felt she was in a strange place, no longer her usual home or territory. In addition, it was probably an uncomfortable place to be in as it was badly damaged by fire. I hate to be critical but why couldn’t she have found a better alternative such as a boarding cattery initially and them something more permanent? Mrs Cotton said that she was unable to settle in a temporary place and was therefore moving around a lot which encouraged her to leave her cat behind. I am sorry, harsh as it might seem, that would not have crossed my mind as an option and I am sure that applies to anyone who genuinely cares deeply about their cat’s welfare.
Tiggy was picked up and taken to a rescue center (Rolvenden Cat Rescue) who scanned her for a microchip and somehow managed to enter the wrong number (one digit was incorrect) in a database which failed to locate Tiggy’s owner (Sam Cotton) as a result of which she was re-homed with another person who now, some weeks later, has formed a bond and does not want to give her up. It seems, on the say-so of the shelter, that Sam Cotton took six weeks to contact the shelter which if true indicates a sloppy attitude to recovering her cat.
Anyway, the story highlights the fallibility of microchipping due to human error. Below are some ways this might happen:
Multiple Registries
In the USA there are many cat registries for microchipped cats. This may cause some confusion. It can lead to mistakes and perhaps a failure to identify a cat’s owner. A very useful search facility can be found on the Pet Microchip Look Up website as it lists almost all the registries. It is open to anyone to use and therefore you can check the registration at any time.
Scan Slowly
I don’t know if this is a genuine or common problem but scanning for the microchip should be conducted slowly and precisely to avoid missing the signal. Multiple scans should be conducted if a chip can’t be found.
Universal Scanners
Chips have evolved and have become more sophisticated. Because of their long life the older ones may not be detected by some scanners. It would seem that universal scanners are a “must have” in order to detect three different frequencies of transponder in the chip. However, universal scanners can still go wrong because e.g. the battery is run down, it needs maintenance or is not the most effective with respect to certain frequencies.
Problematic Scanning
Scanning for a microchip can be a problem, it appears. It is recommended that multiple scanning takes place over the pet’s neck, back and legs. Microchips can move so it is should not be assumed that the chip is in the same place where it was deposited by the vet or technician. Scanners have a circular area which should be placed over the location of the chip but the scanner may work better if the chip is outside the circular part of the scanner. This is problematic; caution and thoroughness might need to be exercised.
Metal
Metal can prevent the scanner detecting the signal from the chip. The metal can be in the cat’s collar or near the cat such as the cage, electrical devices and lighting etc..Does this mean scanning should carried out away from metal objects, even a consulting table?
Defective Microchips
Gail from Boston wrote a neat comment which I quote partially below:
…..We found that out last month when we chipped a number of cats – one cat 4 times – and the chips never registered on the scanner. When we discovered the entire batch was defective (we scanned chips not inserted yet), the vet had to surgically remove all of the defective chips (no easy feat) to be replaced with new ones.
Defective chips are another potential hurdle to accurate microchip identification.
Clearly microchipping is not infallible as a method to ID a cat despite being the most common and successful currently in use. Cat owners should be aware of this and consult with rescue centers accordingly when looking for their cat.