Figuring out the underlying cause of weight loss is very important. In some cases, the cause of weight loss or wasting is obvious, particularly when an opportunistic infection (OI) that is known to cause weight loss has been diagnosed (e.g. MAC or tuberculosis (TB)). Other times, weight loss or wasting can be a symptom of an underlying OI that has not yet been diagnosed. Thus, weight loss that cannot be easily explained often requires that doctors examine their HIV-positive patients carefully, especially if they are losing weight.

After conducting a thorough physical exam, a doctor may have a better sense of the underlying cause of the weight loss. For example, a patient who is losing weight and is also experiencing diarrhea will likely submit stool samples to look for a microorganism in the intestines. Another example is a patient who reports not eating as much upon starting a new anti-HIV medication, indicating that it has affected his or her appetite.

Whether or not a doctor suspects a particularly underlying cause for the weight loss, he or she may order any of the following tests:

	An appointment with a dietitian: If a person's problem might be that he or she is simply not eating enough – or enough of the right foods – meeting with a dietitian can help pinpoint any underlying problems with food intake. If a dietitian finds a problem, he or she will probably recommend diet changes to help manage the weight loss. Meeting with a dietitian can also help control diet problems before they lead to weight loss or wasting.
	Tests to look for problems that directly affect eating: It might be that a sore in the mouth or throat is preventing someone from eating enough food (e.g., thrush, oral hairy leukoplakia, KS, etc.). Another possibility is depression. Tests to look for these medical problems can help figure out the underlying cause of dietary problems.
	A stool sample and/or endoscopy to look for infections of the gut: Infections in the gut can have a serious effect on the way nutrients in food are absorbed into the blood. If an infection is suspected, especially if diarrhea is present, tests can be performed to reach a diagnosis. If a stool test – in which feces samples are sent to a lab for analysis – does not produce any results and an infection is still suspected, a scope can be placed up the rectum or down the throat by a doctor (usually in a hospital setting) to look for any signs of a disease-causing germ.
	Blood tests: Blood tests can say a lot about what might be going on in the body. For example, a doctor should frequently monitor his or her male patients' testosterone levels. A low testosterone in the presence of muscle loss may indicate that hormonal therapy is necessary (discussed next). Blood tests can also help determine if an infection is in the body and possibly causing weight loss.

If an HIV-positive person is losing muscle mass, but does not have any noticeable weight changes, body-composition testing can help figure out if there's wasting going on. One test that is becoming increasingly popular among doctors and researchers is bioelectrical impedance analysis (BIA). This test measures the amount of non-fat parts of the body - called lean body mass (LBM) - including muscle and organ tissue. Depending on the patient's height, weight, and age, BIA can determine if the LBM is low, high, or just about right. A low LBM might indicate muscle wasting. Another test to measure LBM is conducted using a DEXA scan. However, DEXA is more expensive and time-consuming than BIA.

For more information on BIA, click on the following lesson link:

What is a BIA (Bioelectrical Impedance Analysis) Test?