It depends on what they are looking for. Stool thats are pale/white and fatty looking, and tend to 'float' contain excessive amounts of fat due to the fat not being absorbed in the gut. This is often due to problems with the bile production or transport, which is used to emulsify the fats to ease fat absorption. For example, if there was a blockage in the bile ducts in the liver where the bile is made, this would limit/prevent the amount of bile that could be released into the gut. Alternatively, the bile may not be produced correctly, possibly as a result of liver damage or lack of red blood cell death (from which the bile salts can be produced). If the stools are runny then this could be an indication of malabsorption because of infection causing diarrhoea (one of the world's biggest killers). It may also indicate lack of absorption of carbohyrdates or various other biological substances, becaue they would remain in the gut ratyher than being absorbed into the bloodstream, and therefore 'attract' water by exerting an oncotic pressure - in essence keeping water in the gut so that the gut contents don't become too concentrated. Stool samples can be tested for presence of many different food products that would indicate malabsorption, as well as the presence of an infectious organism, similar to both urine and blood samples. Urine tests can test for substances that should not be filtered through the kidneys, such as proteins or large amounts of sugars. This may not indicate a malabsorption of sorts because generally speaking, if products are present in the urine that shouldn't be, this indicates that they have been absorbed at some point into the bloodstream, so is more an indicator of kidney damage. However, it is always worth checking urine for presence of bacteria that could indicate a systemic infection, which can result in malabsorption of food/water in the gut due to inflammation/death of the cells lining the alimentary canal (i.e. the gut wall). Finally blood samples, as with the other two, can be cultured to check for presence of infection. Again, as with the other two, it can also be checked to see if levels of its normal constituents are correct. For example, the level of iron is a relatively common one, to determine if a person is suffereing from anaemia, and can also give clues as to its cause. Abnormal antibody levels, as well as being an indication of infection, can also prove to be useful in identifying auto-immune diseases. These are diseases in which the body's immune system begins to recognise its own normal cells as being harmful and strats to attack them. They are more common in women in general, but men can still develop them. For example, certain types of anaemia are caused by inadequate absorption of vitamin B12. This vitamin is absorbed with the help of a protein called intrinsic factor, secreted by cells in the stomach. If the body begins to think these parietal cells of the stomach are harmful and kills them, the intrinsic factor can no longer be produced and vitamin B12 is not asorbed well, resulting in megaloblastic anaemi - the red blood cells carrying oxygen are swollen and immature. So in summary, there are lots of different tests that can be ran on samples, but these are almost always chosen following a case history and sometimes an examination to determine the signs and symptoms. This leads the doctor to think of several likely possibilities and the tests ordered are to eliminate these or to suggest one possibility is the likely diagnosis.