Spurious blood test results are technically correct but do not match the person's clinical data. Because of this spurious blood test results may be a source of confusion and sometimes harm. The causes of spurious test results from samples drawn from venous lines or by venipuncture may differ. The former arises, essentially, by two mechanisms: 1. dilution of the blood contained in the line by infusate treatment or by fluid used for locking the line, and 2. by adhesion of molecules or particles to the inner surface of the line. Illustrative case histories are presented featuring tests results in blood drawn from venous lines: spurious extreme hyponatremia, spurious steep drop of hemoglobin concentration, and spurious toxic plasma vancomycin. A threefold message emerges from these observations. First, blood samples for analyses should be obtained by direct venipuncture as a norm. Second, when a venous line is already in use it may also serve for blood sampling provided an appropriate fluid volume is discarded from the line before the sample is drawn for analysis. Third, in facing an unusual or improbable laboratory result the test should be double checked by venipuncture. The cause of the inaccuracy should be exposed and avoided in the future.