It is a hemoglobin.
It can go high if you have severe lung disease, but the level needed to 'fail' IMHO is unlikely to be reached easily.
It can go low with bleeding from the gut or cancer, but most people would have other symptoms. In women heavy periods might be a cause
As you can tell I remain to be convinced as to its value. The story I heard some years ago was that the various countries were having the usual argument about which blood tests were needed. The UK said none, Germany said about £1000 worth and others took up opinions in between. As usual, science had no part to play in the negotiations and it was agreed as a compromise one blood test would be included so they chose this one
Not sure whether this is rumour or fact, but a nice story and could well explain it
The test used to be a tube full from the elbow off to the lab. Many AMEs now use a device called a Haemacue which uses a few drops from the finger. We have found it to be merely a guide as opposed to an accurate number in hospitals and use a newer device, again pricking the finger. If anyone did fail I would suggest you ask for a proper lab test if using the Haemacue - might save a lot of tests and delays.....