This question came in through our Ask the Pet Travel Expert feature on PetTravelCenter.com and I thought it was quite relevant to cross-post here:

I am traveling with my pet from India to Seattle via Amsterdam using KLM. There is a layover at the airport for 4 hours at Amsterdam. Will I need a microchip or a blood titre test for the layover? Thanks.

A valid concern:  when traveling with pets internationally, what do you need for layovers in different countries?

The answer is, like most answers about pet travel, somewhat complicated.  It depends on two factors:

1. Whether you're switching airlines (not planes, but airlines).  If you're flying into another country and then changing airlines, then the first airline you are flying will treat your layover city/country as the final destination.  You will be expected to fulfill the import requirements for that country, and clear your pet through customs prior to re-export on your next flight.  This means you would also need all necessary export paperwork for the layover country (another health certificate endorsed in that country, for example).  However, if you're staying on the same airline, and simply changing planes, your final destination is still considered by the airline to be wherever your last flight lands -- you are simply transiting through the layover country with your pet.  In that scenario, you will only need to fulfill the import requirements for your destination country.  Some countries require 'transit permits' to transit through them -- check with the airline you are flying in advance to see if this will be necessary.
2. How long the layover is.  If you are having a layover longer than 24 hours in any country, chances are the country will no longer consider it to be a transit.  You will need to fulfill the import requirements for your layover country in that instance.

As to the question above, since the pet owner is flying on the same airline (KLM) and the layover is only 4 hours, she will not need any additional documents to stop in Amsterdam on her way back flying with pets to the US.

Happy pet travels!