Name Electrolytes Sodium (Na + ) and Potassium (K + ) Flame Photometry, Instrument Principle Any substance, when exposed to sufficiently high temperatures, will be forced into an excited state through thermal collision. Since these states are unstable, the excited atoms or molecules will return to the ground state, dissipating the absorbed energy in various ways, one of which is light emission. Each atom or molecule has associated with it a set of discrete energy levels. In the separated, atomized state, therefore, excited atoms will emit a characteristic set of wavelengths termed a spectrum. The intensity of the light so emitted is directly proportional to the number of atoms undergoing the transition. Thus, by selectively monitoring a characteristic wavelength of an element being volatilized and excited in a flame, the concentration of that element may be measured directly. The alkali metals, Group 1 of the periodic table, have low excitation energies and thus are particularly well-suited for analysis by flame emission. Sodium and potassium can both be analyzed by flame photometry in less time, at lower concentrations, and with greater precision and accuracy than by any other technique. Random changes in flame temperature or aspiration rates and various chemical interferences can cause fluctuations in the emission signal, rendering it fairly unstable. To compensate for this inherent instability in the output, lithium is added in a constant amount to both samples and standards as an internal standard. The 343 electronically compares the signal from the variable sodium and potassium concentrations in the sample to the signal from the constant concentration and displays the ratio of these two quantities directly in units of concentration of the analyzed element. Random fluctuations in the flame thus affect both internal standard and Na & K alike; the ratio remains unaffected, and a stable, accurate readout is obtained. Clinical Significance Sodium Na The body of the average-sized adult contains about 80g of sodium, 35g of which are present in the extracellular fluids. The amount of sodium in the body is relatively constant, despite variation in intake. Although the average person ingests about 3g daily of sodium as the chloride, sulfate, or other salt, he also excretes this amount daily. Since the sodium in plasma is in equilibrium with that in the interstitial fluid, the determination of serum sodium concentration is representative of its extracellular fluid concentration. When an ingested sodium salt is absorbed, there is a temporary increase in extracellular fluid volume, as the absorbed sodium ion equilibrates between plasma and interstitial fluid. There is a small, temporary exchange of sodium for potassium inside the cell. The Summer MLAB 2360! 224
concentration of sodium in the fluid outside the cells is about ten times that of sodium inside the cell. The cell, however, is permeable to sodium ion, and this differential concentration is maintained by a sodium pump. Plasma sodium is filtered in the renal glomerulus and approximately 70% is reabsorbed in the proximal tubule; most of the remainder is absorbed in the distal tubule under the influence of aldosterone, a hormone secreted by the adrenal cortex. Aldosterone accelerates the exchange of Na + for K + across all cell walls including those of the distal tubule. This promotes the retention of Na + and the excretion of K +. The reverse situation occurs if there is a deficiency of aldosterone. Some gonadal steroids may cause a temporary retention of salt and water; this sometimes occurs premenstrually. Increased Concentration (Hypernatremia) Elevated levels of serum sodium are found in (1) severe dehydration owing to inadequate intake of water irrespective of cause, or to excessive water loss; (2) Hyperadrenalism (Cushing's syndrome), in which excessive reabsorption of sodium in renal tubules occurs as a result of over-production of adrenal steroids; (3) comatose diabetics following treatment with insulin as some Na + in cells is replaced by K + ; (4) hypothalamic injury interfering with thirst mechanisms; (5) nasogastric feeding of patients with solutions containing a high concentration of protein, without sufficient fluid intake; and (6) diabetes insipidus (deficiency of antidiuretic hormone) without sufficient intake of water to cover the fluid loss. Decreased Values (Hyponatremia) Most low serum sodium values are found in the following situations: (1) a large loss of gastrointestinal secretions occurring with diarrhea, intestinal fistulas, or in severe gastrointestinal disturbances of any sort; (2) the acidosis of diabetes mellitus before the coma stage, when large amounts of Na + and K + are excreted into the urine as salts of the keto-acids, with replacement of water because of thirst; (3) renal disease with malfunction of the tubular ion-exchange system of Na + for H + and K + (salt-losing nephritis); (4) Addison's disease, with depressed secretion of aldosterone and corticosteroids; and (5) diabetes insipidus (posterior pituitary deficiency) with compensatory intake of water. Potassium K Potassium in the cation having the highest concentration within cells and is approximately 30 times higher than in the extracellular fluids. Approximately 2 to 3g of potassium are ingested and excreted daily in the form of salts. Potassium salts in the diet are absorbed rapidly from the intestinal lumen but have little effect upon the plasma concentration; the rise is slight and transitory. After tissue needs are met, the remainder is excreted by the kidney. The excretion process consists of glomerular filtration, absorption in the proximal tubule, and finally, excretion primarily by exchange for sodium ion in the distal tubules. The kidney does not have the ability to reduce the potassium excretion to nearly zero, as it has for sodium. Summer MLAB 2360! 225
The close control of the concentration of potassium in extracellular fluids is essential because elevated concentrations of K + (>7.0 meq/l) may seriously inhibit muscle irritability, including the heart, to the point of paralysis or cessation of heartbeat. Low serum potassium values are also dangerous because they increase muscle irritability and can cause cessation of the heartbeat in systole (contraction); low serum potassium concentration can be rectified by intravenous injection of appropriate solutions. It is essential for the laboratory to notify immediately the attending physician whenever a seriously high or low potassium value occurs so that appropriate action can be taken in time. The above changes in cardiac muscle irritability caused by either high or low potassium concentration may be reflected in altered electrocardiographic patterns. Increased Concentration (Hyperkalemia) Since the concentration of potassium within cells is so great, its concentration in plasma rises when it leaves the cells at a greater rate than the kidney can excrete it. This overload occurs in conditions of anoxia and acidosis. It also occurs when there is a decreased output of urine, with normal intake of potassium. Conditions of shock or circulatory failure usually produce hyperkalemia. Adrenal cortical insufficiency, particularly a decreased production of aldosterone, is accompanied by an elevation of serum potassium. Elevated serum potassium values commonly accompany chronic renal insufficiency because a tubular malfunction interferes with the exchange of sodium for hydrogen or potassium ion and promotes potassium ion retention. Decreased Concentration (Hypokalemia) A decreased concentration of potassium in serum occurs as a result of either a low intake over a period of time or an increased loss of potassium through vomiting, diarrhea, gastrointestinal fistulas or long-term therapy with diuretics. The fluids of the gastrointestinal tract contain relatively high concentrations of potassium, and their removal or loss can produce serious deficits. Increased secretion of adrenal steroids, primarily aldosterone, results in excessive potassium loss through the kidneys and a low serum potassium concentration. Certain carcinomas that secrete ACTH (adrenocorticotropic hormone) cause a lowering of serum K + concentration through stimulation of the adrenal cortex to produce excessive amounts of steroids. Some diuretics promote K + excretion. Sodium and Potassium in Body Fluids The concentration of Na + and K + in other body fluids is determined by flame photometry or ion-selective electrodes in a manner similar to that in serum; the dilution may have to be modified according to the concentration of these ions. Usually, the Na + concentration in cerebrospinal fluid (CSF), exudates, transudates, and in juices collected from various types of fistulas (pancreatic, duodenal, bile) is within the range of the instrument if treated as serum. The same applies to K +, except that fluid from the ileum may have to be diluted 2 or 3 times more than serum to be in range because values for K + in this fluid may vary from 6 to 29 meq/l. Summer MLAB 2360! 226
In urine, however, the amount excreted depends more directly upon the intake, particularly for Na +. On an average diet, the 24-hour excretion of K + usually varies between 30 and 90 mmol, but it certainly could go higher. For Na + the 24-hour excretion on a usual diet may vary from 40 to 220 mmol, but it could fall to low levels for the patient with a severely restricted salt intake. With some instruments, the urinalysis for Na + and K + can be performed exactly as for serum except that different standards are used, ones that are close to the urine concentration. Normal Values The range of normal values for serum sodium is 135 to 148 meq/l. Since dietary intake of sodium varies there is a wide range of normal values reported for urine sodium. According to Tietz, values given in the literature for normal individual son an average diet vary from 40 to 90 meq/24 hr to 43 to 217 meq/24 hr. Normal serum potassium levels range from 3.5 to 5.3 meq/l in the adult. Urinary excretion of potassium varies greatly with potassium intake but commonly observed levels of persons on an average diet are 30 to 90 meq/24 hr. Acceptable Student Result Range Patient samples should be within normal value range. Abnormal values should be brought to the attention of an instructor. Control samples must be within ± 2 SD. Time Frame 20 minutes Specimen (Sample Collection and Preparation) Typically serum samples are analyzed in the IL 343. To obtain physiologically valid serum sodium and potassium, data, it is important to separate cells from serum as soon as possible after samples have been drawn. If serum remains in contact with the cells for an extended period, the sodium concentration will increase greatly and potassium concentration will decrease. This condition results from an exchange of intracellular materials (refrigeration accelerates the process). Additionally, hemolysis yields false data. If urine samples are to be used, the analysis should be performed on 24-hour urine collections since dietary intake greatly affects the concentration of sodium and potassium in urine. Summer MLAB 2360! 227
Plasma is suitable for analysis if the anticoagulant is a lithium salt. Reagents Sample Preservation Samples should be analyzed as quickly as possible. If samples are to be stored, insure that the container is tightly capped. Care must be taken to prevent contamination of the samples, especially after dilution. Disposable plastic cups are recommended for sample handling since alkali contamination may occur with reusable beakers. Precautions Standard precautions used when handling biological materials. Materials Required But Not Provided Lithium H 2 O Standard Procedure (Sample and Standard Preparation) (Na and K Analyses) Sodium and potassium analyses may be performed on almost any sample with the IL Model 343 Flame Photometer. The following sections detail the procedures necessary for the preparation of such samples and their appropriate blanks and standards. Na & K Serum and Urine Manual Dilutions Non-linearity and inexact results are almost always due to sample contamination or poor quantitative techniques in preparing the dilutions. Quality Control Two levels of assayed control, ie. Monotrol I and Monotrol II References Bishop, M. L., etal. (2000). Clinical Chemistry: Principles, Procedures, Correlations (4th ed.). Philadelphia, PA: Lippincott Williams & Wilkins Summer MLAB 2360! 228
Instruction manual, IL 343 Digital Flame Photometer. Summer MLAB 2360! 229