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Consumers’ Guide to Lab Results
Steve Plogsted, PharmD, BCNSP
One of the many challenges home parenteral and enteral (HPEN) consumers confront is trying to understand the meaning of the tests performed on them. One such set of tests is laboratory values. In HPEN consumers, lab values are frequently used to determine the body’s overall response to artificial nutrition or the effect of a particular disease or condition. Here we will discuss a few of the things an HPEN health care provider might want measured in a blood test: several electrolytes, nutritional markers, and some liver-function indicators.
Before discussing the labs, it is worthwhile to note that lab values can change depending on how and when blood samples are collected and how they are handled. For example, the results of labs drawn during the infusion of parenteral nutrition (PN) could be different from the results of labs drawn just prior to starting the infusion. The most appropriate way to deal with this issue is to consult your physician on the timing of the lab draws. Once a schedule is decided upon, you should follow it consistently in order to obtain the best results. The lab values most affected by the timing are the electrolyte values.
Additionally, the “normal” ranges for each of these tests depend on many factors, such as the age of the patient and the lab where the tests are performed. Labs may have different standards depending on the equipment they use. You need to look at the lab report to determine what those ranges are. When considering the results of the labs, the value may not be as important as how the labs are trending. In other words, if a consumer has an abnormal lab value but it is improving compared to a previous result, the physician may choose not to make changes. It is a good idea to save your lab value reports.
Electrolytes are the salts in the body. They are found both inside of cells (called intracellular salts) and outside of cells (called extracellular salts). Each electrolyte serves a specific role in the body, though often one electrolyte will work in conjunction with other electrolytes to maintain normal body functions, such as muscle tone, heart rate, nerve activity, or even blood pressure. Electrolytes are even involved in maintaining the proper pressure inside of the veins and arteries. When electrolytes go out of balance you may experience symptoms such as fatigue or muscle weakness, low or high blood pressure, headaches, and a variety of other symptoms. Among the major electrolytes are sodium, potassium, calcium, phosphorus, and magnesium.
Sodium is the most abundant extracellular electrolyte in the body. It is mainly responsible for maintaining the proper water balance between various areas or compartments in the body. Ideally, the body maintains sodium levels within a relatively narrow range; at some labs this range is between 135 and 145 mEq/liter of fluid. At these labs, hyponatremia, or low blood sodium, occurs when the value falls below 135 mEq/liter; hypernatremia, or high blood sodium, occurs when the value rises above 145 mEq/liter.
Although many medical conditions can affect sodium balance, the lab values you see are often a reflection of overall body fluid status. High sodium values often mean there is too little body fluid, and low sodium values can mean there is too much body fluid. For example, someone who has had an increase in ostomy output might be losing more water than sodium, causing the sodium value to be elevated. Other causes of low body water status include prolonged fever, a tube feeding formula that is too concentrated, or an inability to consume enough water. Even the presence of a new wound or an increase in the size of a wound can cause significant water loss.
Determining accurate sodium levels is complicated by the fact that high triglycerides and/or high blood glucose can result in a sodium level that is reported lower than it actually is. This is due to the way the lab equipment calculates the sodium. This doesn’t imply that your physician should always order triglycerides or glucose tests when ordering a sodium measurement; it is just a factor to consider if the sodium is abnormal and the cause of the abnormality is not apparent.
The proper treatment for hyponatremia or hypernatremia could involve altering the intake of sodium or controlling the balance of water. Your doctor will make this decision.
Potassium, meanwhile, is the most abundant intracellular electrolyte in the body. Potassium plays a number of important roles in the body. It is involved in maintaining proper heart rate, the conduction of nerve impulses, and muscle contraction. Because potassium is found mainly inside of the cells, blood levels do not adequately reflect total body potassium. By the time your lab values drop noticeably, you may have lost a significant amount of potassium. However, even though blood measurements don’t accurately reflect the body stores, it is extremely important to measure potassium in order to identify trends in the blood. Blood levels are associated with different body responses and can help identify problems occurring in the body. Potassium is eliminated from the body in the urine.
A low blood potassium level (or hypokalemia) occurs in over 20 percent of hospitalized patients and is a common electrolyte abnormality in other clinical practice. Mild hypokalemia is generally not considered a major problem in otherwise healthy individuals, but it can pose significant problems to sicker individuals, such as those with heart disease or those who receive digoxin, a heart medication often used in heart failure. Some consequences of low potassium include generalized weakness, constipation or decreased bowel function, and — if the blood level is low enough — paralysis. Hypokalemia is usually due to abnormal losses, such as those associated with the use of certain medications (diuretics) and increased intestinal fluid output. Low magnesium levels can also result in hypokalemia.
Hyperkalemia (or high blood potassium level) can also occur with the use of certain drugs, and with decreased kidney function. This may occur, for example, after a consumer experiences a large increase in fluid output with inadequate fluid intake. The kidney attempts to regulate the losses by decreasing urine output; this decreases the normal loss of potassium, resulting in high blood potassium. Other causes include the use of certain medications and disturbances of the body’s acid-base status. Some signs associated with high potassium include decreased heart rate, anxiety, abdominal cramps, and diarrhea.
As with sodium, correction of low or high sodium blood level may involve more than just the supplementation or restriction of potassium. Correction may even involve giving magnesium instead of potassium. Again, your doctor will make this decision.
Magnesium is an essential electrolyte involved in metabolism and energy production, the formation of proteins, heart tone, and the activity of nerve impulses. Magnesium works in conjunction with calcium in the balance of muscle contraction and relaxation. It helps regulate the acid-base balance in the body and is crucial in activation of enzymes necessary for carbohydrate, fat, and protein metabolism. Magnesium is also a mineral.
As with potassium, blood levels do not accurately reflect the total body stores of magnesium. Less than 1 percent of the total magnesium in the body is found in the blood and approximately 53 percent is found in the bone. Although the blood levels do not reflect the body stores, we still need to measure the blood levels because we have to follow the trends of the magnesium. Also, as with potassium, blood levels are associated with various body functions and activities, and measuring them can help identify problems occurring in the body. Magnesium is also eliminated from the body in the urine.
Hypomagnesemia (low blood magnesium) occurs from losses mainly from the gastrointestinal tract, as well as a result of some medications. Losses of magnesium can also be the result of intestinal or biliary fistulas, ulcerative colitis, malnutrition, pancreatitis, or vomiting. When someone has low magnesium, one of the difficulties of replacing the magnesium with oral medications is that these magnesium salts can themselves cause diarrhea. Common effects of low magnesium include weakness, confusion, difficulty swallowing, and growth failure in children.
Hypermagnesemia (high blood magnesium), when the kidneys are healthy, occurs primarily as a result of increased intake. Excess intake can occur from using magnesium-based laxatives. Common effects of high magnesium include low blood pressure (hypotension), flushing, and even difficulty breathing.
It is important to understand that low blood levels of magnesium can affect the calcium balance in the body. Magnesium is required to make parathyroid hormone, which regulates calcium in the body. Also, as mentioned earlier, low magnesium blood levels can cause low blood potassium.
Calcium is a major component of bones and is involved in the maintenance of blood pressure and blood clotting. When calcium is included on a laboratory report it is generally reported as total blood calcium. Calcium is highly bound, mainly to albumin (an important protein in the blood; discussed below), and the remainder floats free. The biological activity in the blood is related to the free calcium. When a person has a low albumin level, the measured calcium will also be low; however, the free portion may remain normal.
There are numerous formulas that can be used to calculate the corrected level of calcium in the blood. The one used most often in clinical practice follows. (Note that although the normal albumin level ranges from 3.5 to 4.5 gm/dl, we will use a value of 4 gm/dl in our calculation, which is what many physicians do. Also, for the example in Figure 1, we will use 3 gm/dl as the measured albumin level and 7.5 as the measured calcium.)
The formula: Assuming that the normal albumin level is 4 gm/dl, for every 1 gm/dl that the albumin is below 4 (see Figure 1, A) you correct the calcium in the opposite direction by a factor of 0.8 (see Figure 1, B and C).
As mentioned above, if you have low calcium (hypocalcemia) your magnesium may be out of balance. Your health care provider should ensure the magnesium levels are normal and are not contributing to the hypocalcemia. Some other causes of low calcium include the use of certain diuretics (the water pills like furosemide), low vitamin D intake, and a high intake of phosphorus. Some common symptoms of hypocalcemia include numbness, breathing difficulties, and muscle twitching or tightness.
Symptoms of high blood calcium (hypercalcemia) include weakness, fatigue, high blood pressure, nausea or anorexia, and decreased muscle response. Causes of high calcium include excess intake of calcium or vitamin D, calcium-containing antacids, and another diuretic called hydrochlorothiazide.
Phosphorus is another electrolyte that is important to bone health, the body’s acid-base balance, and the production of energy. It is also a component of cell membranes. Once again, phosphorus is also mainly found inside of the cells and blood levels do not reflect total body stores.
Your bones serve as a reservoir for phosphorus. Low blood phosphorus levels will cause a loss from the bones, since maintaining blood levels is more important to the body than maintaining bone health. Phosphorus and calcium are dependent on each other, and both are controlled by hormones. Aggressive replacement of either electrolyte can cause a change in the blood level of the other, so care must be taken when treating low levels of either one.
Causes of low phosphorus include low intake of phosphorus and the use of calcium-containing antacids. High levels often occur due to poor kidney function or excess intake of certain laxatives.
There are no blood values or levels that measure nutritional status. However, there are some blood proteins and enzymes that may alert the physician to changes in the body in response to artificial nutrition. These markers are only one tool that the physician uses in the assessment of the consumer’s overall health. Although there are a number of markers, I will cover only a few here.
Albumin is a protein found in the blood; it is responsible for maintaining proper fluid balance inside the system that circulates the blood in the body (similar action to sodium). It is also involved in carrying various materials and drugs to different body locations. It is made in the liver, and proper nutrition is one mechanism necessary for its production. Historically, low albumin was considered an indication of malnutrition, but we know now that this is not entirely true.
Interpreting blood albumin levels is a complicated and complex process. Low blood albumin levels can be the result of wasting in certain bowel diseases, malnutrition, or the presence of illness. An otherwise healthy person who has not been taking in proper nutrition for an extended period of time can experience a decline in albumin. More often, low albumin is the result of some sort of ongoing disease process. Fistulas, open sores, recent serious fractures, and other stress can cause a decline in albumin levels. A significant increase in calories can actually cause more harm than good.
Albumin is slow to return to normal after a decline and should not be used as a marker in acutely ill individuals. A protein with a faster recovery time, such as prealbumin, would be a better marker. Albumin and prealbumin levels should never be used alone when assessing a person’s nutritional status but should be used in conjunction with other measurements.
During an acute illness or if some other inflammatory process is occurring, another blood protein can also be helpful. This protein, called C-reactive protein (CRP) is increased in the blood in response to stress. When stress occurs and the CRP rises, albumin and prealbumin levels generally fall. This is a natural process as the body responds by manufacturing CRP in preference to the other blood proteins. As the stress resolves, the other blood proteins are manufactured and levels return to normal.
Liver Function Tests
As with nutritional markers, liver function cannot be measured by blood tests. The physician can, however, use blood levels of enzymes produced by the body to monitor the effect of disease or nutrition on the liver.
One of the liver enzyme lab values that often has hidden meaning is alkaline phosphatase (ALP). The liver is one of the sites where the enzyme is manufactured, and in liver disease, the ALP will rise. However, ALP is also produced in the bone. The laboratory measurement doesn’t distinguish between the two and reports all forms of ALP. Persons with bone disease or cancer, for example, will also have an elevated ALP. Physicians can order specific ALP measurements to identify the source, but that is not frequently done.
When ALP is manufactured it requires zinc to be incorporated into the enzyme. People with zinc deficiency will have low levels of ALP. Zinc deficiency can occur in patients with high stool output, short bowel syndrome, Crohn’s disease, burns, and fistulas. Interpreting ALP also requires evaluation of these other conditions to see if they are possibly contributing to the ALP levels.
Bilirubin is a byproduct of the breakdown of hemoglobin in red blood cells. It is carried to the liver where it is processed by the liver (direct bilirubin). The direct bilirubin is released into the bile and also stored in the gallbladder. When this process is impaired, the direct bilirubin is elevated.
There are several causes of this elevation, including gall stones, fat deposits in the liver, cirrhosis, and hepatitis. The fat deposits can occur from too much fat in the diet, too much carbohydrate, or just too many calories. Other causes of fatty liver include the use of certain drugs and severe or significant infections. Your physician will consider all of these factors before making any decisions on a course of action.
Evaluating laboratory values is an art, as well as a science. It demands total attention to all the factors that can contribute to the various measurements. It is important to note that not everything is always as it seems. Simply looking at where a lab value falls in relation to the normal range may not give you an accurate picture of what is happening.
Evaluating lab values should be left up to your physician. It is good, however, for HPEN consumers to understand the value of the testing.