Yes, it can. Of course, there are many variables involved. It depends on the test and the actual condition of your body after the workout. For the most accurate results, blood tests should be done when rested or with normal activity. Some are only accurate when you have been fasting. Your doctor will let you know whether there are any special instructions. You should ask the technician to note any special circumstances.
Qualitative vs. Quantitative Tests
Most of the tests ordered for a routine physical or a simple illness are either qualitative or quantitative. Qualitative tests are seeking the presence of a substance at or above a specific diagnostic level. Quantitative tests seek the precise amount of a substance. Blood cell counts are similar to a quantitative test in that the presence of blood cells is known, so the number of each type is what is tested. If a qualitative blood test is positive for a substance, a quantitative test may follow to determine the amount, notes MedlinePlus.
Dehydration may follow a vigorous workout. It is unlikely to affect a qualitative test, but it might skew a quantitative test. If you are dehydrated, the number of blood cells present in the sample may be more than usual because they are in less fluid. You might appear to have a healthy number of red cells when you do not. However, a greater number than normal of all cells tend to indicate your dehydration. The test may have to be done again to get results that are more accurate.
Over-hydration does the opposite. If you have consumed a lot of fluids after your workout, you might seem to have fewer red cells than is normal. They are diluted by the increased amount of blood. Extreme dilution will be noted by a lower-than-normal number of all blood cells. As with dehydration, the test may have to be repeated to get accurate results. Qualitative tests may be affected, but there are safeguards in drug testing to assure that dilution is not a factor. This is also true of urine tests, where specific gravity testing can indicate hydration status.
There are enzymes released with muscle activity during a workout. If you are also dehydrated after a workout, the enzyme CK, also called CPK, may be elevated enough to cause concern. It may be only temporary from your exercise, or it may indicate a muscle or heart condition. The potentially serious muscle condition rhabdomyolysis causes extremely high CK levels and severe muscle pain. Damage to the heart muscle will also cause elevated CK. According to Lab Tests Online, this test is not part of the usual routine blood test panels, but it is done when muscle damage is suspected. A workout before a fasting blood test can alter the results of cholesterol and glucose tests, according to Johns Hopkins Health Alerts.
Blood tests take time, cost money and cause discomfort. It is advisable to have them done when you are well rested or as instructed by your doctor or the lab. If for some reason you must have routine tests done after a workout, tell the technician and your doctor so they can interpret the tests correctly. If there is doubt, you may need to have the tests done again. It is better to save yourself the worry and expense by delaying your workout.