A Runnerís Guide to Interpreting
by Cathy Fieseler, MD
There's a message on your voicemail from your physician, saying that you need to set up an appointment to discuss abnormal results from lab work that was performed as part of a recent physical exam. Immediately your mind starts racing. Is something seriously wrong? Could the hilly half marathon that you ran the day before the exam have affected the results?
Regular distance running can cause changes to some commonly measured blood values, both in the short term and long term. Let's look at some of those changes, including both ones that can be cause for concern and ones that you needn't worry about.
Anemia and Red Blood Cells
Anemia is a fairly common problem for runners and non-athletes alike, especially females. A diagnosis of anemia is due to a decreased number of red blood cells (RBCs) or a decreased hemoglobin level. There are a number of types of anemia, with iron deficiency anemia the most common. (Iron is a component of hemoglobin, the oxygen-carrying component in RBCs.)
Iron deficiency anemia is due to nutritional deficiencies and blood loss. Small amounts of blood are lost in the gastrointestinal tract on a daily basis; the blood loss may increase during and following a strenuous bout of exercise. Due to menstrual blood loss, women normally have lower hematocrit and hemoglobin levels than men. Anemia doesn't always cause symptoms, but it may impair performance. This is treated by finding the cause of the anemia and supplementing iron intake.
Regular exercise causes an increase in the number of RBCs in the blood. As an adaptation to training, there's also an increase in plasma volume in the resting state. This volume expansion causes the hematocrit (the percentage of RBCs in blood) and hemoglobin levels to be lower than in non-athletes. The expansion in plasma volume will be reflected as lower hematocrit and hemoglobin levels on a complete blood count (CBC). For most runners, hemoglobin will be approximately 0.5g/dl lower than normal, while it can be 1g/dl lower in highly trained runners. This lower level is a pseudoanemia and doesn't require medications. The dilutional effect of training will resolve after several days off from training.
"Abnormal" Lab Results
An acute bout of exercise can cause a number of "abnormal" laboratory results. A urine test may be positive for blood. This may be due to bleeding from the bladder or kidneys; this is usually not visible to the naked eye, but is detectable in the lab. This resolves within 24 to 72 hours after exercise. If blood is still found on subsequent testing, further evaluation is indicated.
Protein may be detected in the urine within 30 minutes of strenuous exercise. This will clear within 24 to 48 hours. (Persistent signs of this condition warrant further evaluation.)
A positive test for blood in the urine may be due to myoglobin released from muscles injured during exercise; when evaluated under a microscope, red blood cells aren't seen. When myoglobin levels are high, the urine will be dark in color, like iced tea. Blood levels of myoglobin and the muscle enzyme creatine kinase (CK) will be elevated. Strenuous exercise in conjunction with dehydration may produce very high levels of CK in the blood and myoglobin in the blood and urine, a condition known as rhabdomyolysis. This can cause renal failure and may be fatal in severe cases. Blood levels of potassium and phosphorous may be high and calcium levels low in severe cases of rhabdomyolysis.
Blood levels of CK are elevated following exercise. Many factors will affect the level of elevation, including the length and intensity of the exercise. Levels of CK (normal < 200U/L) greater than 200,000 U/L have been reported following the Western States 100-miler. Most athletes will have levels much lower than this following exercise.
Recovery from the workout and good hydration will usually take care of this issue. CK levels will gradually return to normal over the next several days to a week. During bouts of heavy training, the level may remain mildly elevated chronically. The cholesterol-lowering group of medications known as statins may also increase the CK, even at rest. This can be greatly, and potentially dangerously, elevated by strenuous exercise.
Dehydration associated with exercise may cause elevation of BUN and creatinine, two aspects of a kidney test. The BUN may be markedly elevated in the presence of severe dehydration; the creatinine may be mildly elevated. Rehydration will normalize these values.
AST is one of the tests that evaluates liver function. This enzyme is also present in skeletal muscle; levels will be elevated following exercise. The other tests that are part of a typical liver function evaluation aren't significantly affected by exercise.
A small amount of blood is lost from the gastrointestinal tract on a daily basis; this isn't visible to the naked eye. Strenuous exercise may increase the amount of blood lost for a day or two afterwards; although this is usually not visible, it may be detected on stool cards that are used to screen for colon cancer.
A CBC evaluates the components that make up blood -- red cells, white cells, platelets and plasma. Following an acute bout of exercise, there's a transient increase in the white blood cells and platelets. The hematocrit will be increased from baseline when you're dehydrated.
It's best to wait a few days following strenuous exercise to have laboratory studies performed to lower the possibility of a result that's altered by exercise. The alteration in lab studies as a result of exercise should resolve within a few days. The first step in evaluating abnormal results may be to repeat the studies after several days of minimal training.
Lab Results That Point to Performance
What laboratory studies are routinely evaluated during a physical exam that may affect performance? Blood chemistries will include electrolytes, glucose and kidney function tests. A more comprehensive evaluation will also assess liver function, in addition to calcium and uric acid. Other than the AST (liver) and CK levels, exercise shouldn't have a significant effect on chemistry results. Abnormal results may be due to liver disease, kidney disease, diabetes and other health issues, any of which can hurt performance and require treatment. Electrolyte abnormalities are usually due to altered hydration levels and should normalize as you rehydrate.
A CBC is often performed; as mentioned, the hematocrit and hemoglobin levels may be a little low due to plasma volume expansion. The volume expansion resolves if training is discontinued for several days. Ferritin is commonly used to assess iron stores; if this is low and the hematocrit and hemoglobin levels are low, treatment for iron deficiency anemia may be called for. Anemia may cause a decrease in cardiac output and the oxygen available to tissues, negatively affecting performance.
A low ferritin level with a normal CBC is consistent with low iron stores without anemia. This condition doesn't affect performance, but may progress to anemia over time; improving intake in the diet and possibly supplementing with iron tablets may prevent progression. It's important to determine why the anemia is present. In women, poor dietary intake and menstrual blood loss are the most common causes. In men, gastrointestinal loss is the most common source.
Thyroid tests may be performed as part of a physical exam. A thyroid that's under-or overactive can cause multiple symptoms and negatively impact performance.
Although a high cholesterol level will not specifically affect performance, coronary artery disease due to elevated lipids can. Coronary artery disease may cause fatigue with exercise and worsening performance despite no change in training. Don't ignore these symptoms!
There has been a great deal of research on vitamin D levels recently, and blood levels are being checked more routinely now. Due to use of sunscreen, poor dietary intake and other factors, low levels are fairly common. A multitude of symptoms have been attributed to low levels of vitamin D, including unexplained muscle pain and an increased risk of stress fractures. Supplementing intake will correct this deficit.
It's important to keep in mind that there are many possible causes of laboratory test anomalies, only a few of which I covered above. Although strenuous training may be the cause of the results, other causes must be considered. Don't ignore recommendations for further evaluation made by your healthcare provider; if the laboratory studies were performed shortly following strenuous exercise, mention this to your provider. Decreasing training and repeating the studies in several days may be all that is necessary.
Cathy Fieseler, MD, is the president of the American Medical Athletic Association.