Blood tests and urine lab tests are very common, and the results are generally very reliable. But it may happen that the result is a so-called “false positive”. This means that they signal a problem when in reality there isn’t one.
Dr. Roberto Colombo, laboratory coordinator in Humanitas, spoke in an interview on Starbene regarding the matter.
“The values of creatinine, a waste substance normally eliminated in the urine, should normally be between 0.5 and 1.2 mg/dl. During bouts of diarrhea or after excessive sweating when your body is dehydrated, the test may be distorted.
The risk of a false-positive is also present in patients suffering from Gilbert jaundice. This is a slight congenital disorder where the urine sample reacts with a particular reagent (Jaffè) in the laboratory. Even with azotemia the result can be reliably positive (with values greater than 40 ml/dl) if the patient has consumed liquids or has had too much protein (e.g. meat or dairy products) in the 24 hours before the test”.
CEA, PSA and Ca 19.9
CEA is Carcino-Embryonic Antigen, a glycoprotein whose levels in the blood tends to rise in the presence of certain cancers. Particularly those of the colon, rectum, lung and breast. “The CEA is normally high in smokers without this signaling the presence of a tumor,” explains Dr. Colombo.
Ca 19.9 is a marker of pancreatic cancer . “The values in these tests are raised in those who suffer from inflammatory bowel disease (such as Crohn’s disease and ulcerative colitis) or chronic bronchitis. False positives can also be waiting to happen depending on the reagent that is used in the laboratory. For this reason, these tumor markers should not be used to assess the risk of cancer or, as often happens, for their early diagnosis. Instead, they are useful when tumors have already been identified, to assess the effects of care. If their values are lowered, we can see if the therapy is effective”
The dosage of the PSA, or prostate-specific antigen is administered to check the health of the prostate. The outcome is considered positive if the value is greater than 4 ng/ml. Explains Dr. Colombo: “In 7 out of 10 cases these results do not indicate a cancer of the male genital gland. The PSA can be high due to an inflammation of the prostate, especially in the elderly. They can also be high after a long ride, after an andrologic visit with rectal examination or if you have had sexual intercourse with ejaculation on the day before the exam.”
The HIV test
During an HIV test, if the patient is pregnant, the probability of a false positive is very high, especially in the first three months of gestation. “The presence of the baby in the belly triggers a kind of chaos in the body of the expectant mother and numerous immunological factors and antigenic variations in the blood may cause a false positive for the AIDS test. Before being alarmed it is good to make a confirmatory test, where we measure antibodies against HIV in a more precise and specific test not likely to give a false positive”.