Chemistry - Electrolytes and Osmolality
Which of the following is a spectrophotometric method for quantifying serum chloride? A. Ferric perchlorate B. Ammonium molybdate C. Bathophenanthroline D. Cresolphthalein complexone
A. Chloride can be quantified by the spectrophotometric ferric perchlorate method. The reagent reacts with chloride to form a colored complex. Other methods employed are the spectrophotometric mercuric thiocyanate method, the coulometric-amperometric titration method, and the ion-selective electrode method, which is employed by many automated analyzers.
Which of the following has an effect on plasma calcium levels? A. Sodium B. Inorganic phosphate C. Potassium D. Iron
B. Plasma phosphates influence plasma calcium levels. Case studies show that there is a reciprocal relationship between calcium and phosphorus. A decrease in plasma calcium will be accompanied by an increase in plasma inorganic phosphate.
What is the primary storage form of iron? A. Apotransferrin B. Myoglobin C. Ferritin D. Hemosiderin
C. In adults the total body iron content averages 3-4 g. The majority of this iron is found in the active pool as an essential constituent of hemoglobin, with a much lesser amount being an integral component of myoglobin and a number of enzymes. Approximately 25% of the body iron is found in inactive storage forms. The major storage form of iron is ferritin, with a lesser amount being stored as hemosiderin. Ferritin may be found in most body cells but especially in reticuloendothelial cells of the liver, spleen and bone marrow.
Of the total serum osmolality, sodium, chloride and bicarbonate ions normally contribute approximately what percent? A. 8 B. 45 C. 75 D. 92
D. Osmolality is a measure of the total number of solute particles per unit weight of solution and is expressed as milliosmoles per kilogram of water. The normal osmolality of serum is in the range of 275-295 mOsm/kg water. For monovalent cations or anions the contribution to osmolality is approximately 92%. Other serum electrolytes, serum proteins, glucose and urea contribute to the remaining 8%
In the collection of plasma specimens for lactate determinations, which of the following anticoagulants would be more appropriate? A. Sodium heparin B. Sodium citrate C. EDTA D. Oxalate plus fluoride
D. Plasma lactate concentrations are increased in cases of lactic acidosis. The accumulation of lactate in the blood results from any mechanism that produces oxygen deprivation of tissues and, thereby, anaerobic metabolism. Lactate concentrations in whole blood are extremely unstable because of the rapid production and release of lactate by erythrocytes as a result of glycolysis. One method of stabilizing blood lactate levels in specimen collection is to add an enzyme inhibitor such as fluoride or iodoacetate to the collection tubes. Heparin, ethylenediaminetetra-acetic acid (EDTA), and oxalate will act as anticoagulants but will not prevent glycolysis in the blood sample.
Which of the following is not a component of the total anion content of serum? A. Acetoacetate B. Protein C. Lactate D. Iron
D. The largest fractions of the anion content of serum are normally provided by chloride and bicarbonate. The third largest anion fraction is contributed by the proteins that are negatively charged at physiological pH and that provide about 16 mmol ion charge per liter. Of the remaining organic anions, the largest contribution is generally from lactate, which ranges normally form 1 mmol/L up to 25 mmol/L in lactic acidosis. The ketone bodies, including acetoacetate, normally constitute only a small fraction of the total anions, but their total contribution may increase to 20 mmol/L in diabetic acidosis. Iron is present in the serum as a cation and does not contribute.
Which of the following is not a typical finding in magnesium deficiency tetany? A. High serum phosphate level B. Normal serum calcium level C. Normal blood pH value D. Low serum potassium level
A. A low ionized serum magnesium level is characteristic of a magnesium deficiency tetany. The serum magnesium level usually ranges between 0.15 and 0.5 mmol/L when tetany occurs. In addition, the serum calcium level and blood pH are normal whereas the serum potassium level is decreased. This type of tetany is treated with MgSO4 to increase the level of serum magnesium, thus alleviating the tetany and convulsions that accompany this disorder.
Which of the following is not a colligative property of solutions? A. pH B. Freezing point C. Osmotic pressure D. Vapor pressure
A. Colligative properties of a solution are those properties that depend only on the number of particles in solution, not on the nature of the particles. The colligative properties are boiling point, freezing point, osmotic pressure and vapor pressure. Terms used to describe the concentration of particles in solution are "osmole" (the number of particles, 6.0224 X 10^23, that lowers the freezing point 1.86 degrees Celsius) and "osmolal" (a concentration of 1 Osm of solute per kilogram of water). One mole of an un-ionized solute dissolved in 1 kg of water lowers the freezing point 1.86 degrees Celsius. Thus it is an osmolal solution. For un-ionized substances such as glucose, 1 mol equals 1 Osm. For substances that ionize, such as sodium chloride, wherein each molecule in solution becomes two ions and thus two particles, 1 mol of sodium chloride theoretically equals 2 Osm. In reality, however, this is not always the case; an osmotic activity coefficient factor is used to correct for the deviation. In practice, three types of osmometers are available. They are the freezing point, vapor pressure, and colloid osmotic pressure osmometers.
Which of the following constituents normally present in serum must be chemically eliminated so that it will not interfere with the measurement of serum magnesium? A. Calcium B. Chloride C. Iron D. Potassium
A. Magnesium measurements are commonly done spectrophotometrically using reagent systems such as calmagite, methylthymol blue and chlorophosphoanzo III. Calcium will interfere and is eliminated by complexing with a chelator that binds calcium and not magnesium. Atomic absorption is a specific and sensitive method for analysis of magnesium, with the only significant interference being phosphate ions, which are removed by complexing with a lanthanum salt.
The following laboratory results are obtained on a 60-year-old woman who is complaining of anorexia, constipation, abdominal pain, nausea and vomiting: Ionized serum calcium -> elevated Serum inorganic phosphate -> decreased Urine calcium -> elevated Urine phosphate -> elevated What so these results suggest? A. Primary hyperparathyroidism B. Vitamin D deficiency C. Hypoparathyroidism D. Paget disease
A. Primary hyperparathyroidism is a disorder characterized by increased secretion of PTH into the blood, without the stimulus on the parathyroid gland of a decreased level of ionized calcium. The increase in PTH produces increased blood calcium and vitamin D3 levels, along with a decreased blood phosphorus level. The hypersecretion is most often caused by a single parathyroid adenoma. PTH secretion can usually, but not in all cases, be suppressed by calcium infusion. The decreased blood phosphate level is a result of the action of PTH on the kidneys, which decreases tubular reabsorption of phosphate ions. The increased blood level of 1,25-dihydroxyvitamin D3 is also caused by PTH action on the kidneys in that PTH stimulates increased renal synthesis of this compound.
Using the following data: Na+ =143 mmol/L; K+ = 4.9 mmol/L; Cl- =105 mmol/L; and HCO3- =25 mmol/L, which of the following statements is false? A. Patient results are not acceptable B. Anion gap is useful in detecting some disease states C. Anion gap equals 18 mmol/L D. Anion gap is useful for checking analytical error.
A. The calculation of the anion gap may be used both to assess instrument performance and as a quality assurance tool for electrolyte analyses. The following is one of several equations that may be used to calculate the anion gap: anion gap (mmol/L)= (Na+ + K+) - (Cl- + HCO3-). The acceptable reference for this method of calculation is 10-20 mmol/L. If the values of a particular patient fall within this acceptable level, it is presumed that there are no gross problems with the electrolyte measurements. In this case, the anion gap is 18 mmol/L and within the reference range. When using the anion gap it is important to remember that values are affected not only by measurement errors but also by such disease processes as renal failure, ketoacidosis nad salicylate poisoning. Therefore, it is important to differentiate between laboratory errors and true disease states.
Which of the following reagents is used in a colorimetric method to quantify the concentration of serum calcium? A. Cresolphthalien complexone B. Lanthanum C. Malachite green D. Amino-naphthol-sulfonic acid
A. Total serum calcium concentration is often determined by the spectrophotometric quantification of the color complex formed with cresolphthalein complexone. Magnesium will also form a color complex and, therefore, is removed by reacting the serum with 8-hydroxyquinoline. Calcium concentration is determined with the use of a variety of other reagents and most reliably by means of atomic absorption spectrophotometry.
In iron-deficiency anemia, what would be the expected percent saturation of transferrin with iron? A. Less than 15 B. Between 30 and 40 C. Between 40 and 50 D. Greater than 55
A. Transferrin is a glycoprotien that reversibly binds serum iron that is not combined with other proteins such as hemoglobin and ferritin. Transferrin concentration in serum is rarely determined directly but, rather, in terms of the serum iron content after saturation with iron. This is the total iron-binding capacity (TIBC). The percent saturation of transferrin is determined by dividing the serum iron level by the serum TIBC and expressing this value as a percentage. Normally in adults the percent saturation of transferrin is in the range of 20-50%, whereas in iron-deficiency anemia, the saturation is expected to be less than 15%. In iron-deficiency anemia complicated by other disorders that either increase serum iron concentration or decrease the TIBC , the percent saturation may remain within the reference range.
In what disorder would an increased percent saturation of transferrin be expected? A. Hemochromatosis B. Iron-deficiency anemia C. Myocardial infarction D. Malignancy
A. Transferrin is the iron transport protein in serum and is normally saturated with iron to the extent of approximately 20-50%. An increased percent saturation of transferrin is expected in patients with hemochromatosis, an iron overload disease, and iron poisoning. The increased saturation is due to the increased iron concentration in the serum. In patients with chronic infections and malignancies, there is impairment of iron release from body storage sites, leading to a decreased percent saturation of transferrin. In myocardial infarction the serum iron levels are depressed, but the TIBC levels are normal. Iron-deficiency anemia because of poor absorption, poor diet or chronic loss results in decreased serum iron, increased transferrin and decreased percent saturation of transferrin in most cases.
To what metal does ceruloplasmin firmly bind? A. Chromium B. Copper C. Zinc D. Iron
B. Copper is found in the plasma mainly in two forms: a minor fraction loosely bound to albumin and the majority, representing about 80-95%, firmly bound to the enzyme ceruloplasmin, an alpha2-globulin, which is important in the oxidation of iron from the ferrous to the ferric state. Copper is also an essential constituent of a variety of other enzymes found in erythrocytes and in other sites throughout the body. The major clinical usefulness of determining serum copper or ceruloplasmin levels is that the decreased level of both is associated with Wilson disease. Decreased levels of copper are also found in protein malnutrition and malabsorption and in nephrosis.
The presence of only slightly visible hemolysis will significantly increase the serum level of which of the following electrolytes? A. Sodium B. Potassium C. Chloride D. Bicarbonate
B. Hemolysis of blood specimens because of physiological factors is often difficult to differentiate from hemolysis produced by the blood collection itself. In either case, the concentration of potassium will be increased in the serum because of the release of the very high level of intracellular potassium from the erythrocytes into the plasma. When hemolysis is present, the serum concentrations of sodium, bicarbonate, chloride and calcium will be decreased because their concentrations are lower in erythrocytes than in plasma.
Which of the following reagents is used to determine the concentration of serum inorganic phosphate? A. Ehrlich's reagent B. Ammonium molybdate C. 8-Hydroxyquinoline D. Bathophenanthroline
B. Serum inorganic phosphate concentrations are determined most commonly by reacting with ammonium molybdate reagent. The molybdenum-phosphate complexes can be quantified at 340 nm. Alternately, treatment of the phosphomolybdate compound formed with a reducing agent leads to the formation of molybdenum blue, which can be measured spectrophotometrically. Use of the anticoagulants EDTA, oxalate and citrate should be avoided, because they interfere with the formation of phosphomolybdate.
Which of the following chromogens willnot produce a colored complex with iron that can be measured spectrophotometrically? A. Bathophenanthroline B. 8-Hydroxyquinoline C. Tripyridyltriazine D. Ferrozine
B. Serum iron concentrations are most often determined by the colorimetric reaction with ferozine, bathophenanthroline, or tripyridyltriazine. The same reagent is usually used in the determination of serum TIBC by saturating the transferrin in the serum with an excess of iron, removing any unbound iron, and measuring the iron bound to transferrin. This measurement of TIBC provides a measure of transferrin concentration. Several magnesium methods require the precipitation of magnesium as part of the analysis, and 8-hydroxyquinoline effectively precipitates magnesium.
A patient's serum inorganic phosphate level is found to be elevated but the physician cannot determine a physiological basis for this abnormal result. What could possibly have caused an erroneous result to be reported? A. Patient not fasting when blood was drawn B. Specimen was hemolyzed C. Effect of diurnal variation D. Patient receiving intravenous glucose therapy
B. Similarly to potassium, which is a major intracellular cation, phosphate is a major intracellular anion. Therefore, when blood is drawn for serum inorganic phosphate measurement, hemolysis of the specimen must be avoided. Also, serum should be removed from the clot as soon after collection as possible to avoid leakage of phosphate into the serum. Both of these situations would contribute to falsely increased phosphate levels. Conversely, serum phosphate levels will be depressed following meals, during the menstrual period, and during intravenous glucose and fructose therapy.
A patient presents with Addison disease. Serum sodium and potassium analyses are preformed. What would the results reveal? A. Normal sodium, low potassium levels B. Low sodium, low potassium levels C. Low sodium, high potassium levels D. High sodium, low potassium levels
C. Addison disease is characterized by the hyposecretion of adrenocortical hormones by the adrenal cortex. Both aldosterone, a mineralocorticoid, and cortisol, a glucocorticoid are inadequately secreted in this disorder. The decreased secretion of aldosterone will affect body electrolyte balance and extracellular fluid volume. The decrease in sodium reabsorption by the renal tubules will be accompanied by decreased chloride and water retention. This loss of sodium, chloride and water into the urine will cause the extracellular fluid volume to be decreased. Additionally, the decreased reabsorption of sodium will interfere with the secretion of potassium and hydrogen ions in the renal tubules, causing an increase in the serum potassium ion and hydrogen ion (acidosis) concentrations.
Which of the following statements is false about the electrolyte chloride? A. Main anion of the extracellular fluid B. Can shift from the extracellular plasma to the intracellular fluid of red blood cells C. Unable to be reabsorbed by active transport D. Measured in serum, urine and sweat
C. Chloride is the principal plasma anion. The average concentration of chloride in plasma is 103 mmol/L. In the kidneys, chloride ions are removed from the blood through the glomerulus and then passively reabsorbed by the proximal tubules. The chloride pump actively reabsorbs chloride in the thick ascending limb of the loop of Henle. In the lungs, chloride ions participate in buffering the blood by shifting from the plasma to the red blood cells to compensate for ionic changes that occur in the alveoli when the HCO3- from the red blood cells enters the plasma. This is termed the chloride shift. Chloride can be measured in a variety of body fluids, including serum, plasma, urine and sweat.
Of the total serum calcium, free ionized calcium normally represents approximately what percent? A. 10 B. 40 C. 50 D. 90
C. Free ionized calcium normally accounts for about 50% of total serum calcium, with the remainder being made up of complexed calcium (about 10%) and calcium bound to proteins (about 40%). The main factors that affect the free ionized calcium fraction are the protein concentration and the pH of the blood. Calcium ions are bound mainly to albumin, but they also bind to globulins. Because the binding is reversible, factors that decrease the protein concentration will increase the free ionized fraction of calcium in the blood. A decrease in blood pH will also increase the fraction of free ionized calcium.
Which of the following disorders is best characterized by these laboratory results? Serum iron -> decreased Total iron-binding capacity -> increased Transferrin saturation -> decreased Serum ferritin -> decreased Free erythrocyte protoporphyrin -> increased A. Anemia of chronic disease B. Thalassemia C. Iron-deficiency anemia D. Hemochromatosis
C. In order to differentiate among diseases, it is necessary to perform several laboratory determinations to properly assess iron metabolism. In iron-deficiency anemia, the serum iron is decreased whereas the TIBC is increased. Thus it follows that the transferrin saturation is decreased.The serum ferritin level, which represents stored body iron, is depressed, and the free erythrocyte protoporphyrin (FEP) level is increased. FEP is not a specific test for iron-deficiency anemia, but it can function as a screening test.
Which of the following disorders is characterized by increased production of chloride in sweat? A. Multiple myeloma B. Hypoparathyroidism C. Cystic fibrosis D. Wilson disease
C. Measuring the concentration of chloride in sweat is a commonly used diagnostic procedure for determining the disorder of cystic fibrosis (CF). The majority of patients with CF will present with increased concentrations of sodium and chloride in their sweat. Generally, children with CF will manifest sweat chloride levels that are two to five times the reference interval. In sweat testing, sweat production is stimulated by iontophoresis with pilocarpine. Then the sweat is either collected and analyzed for chloride or an ion-selective electrode is applied to the skin surface to quantify chloride. It has been established that the gene abnormality causing CF is located on chromosome 7.
Secondary hyperparathyroidism is often the result of A. Vitamin C deficiency B. Liver disease C. Renal disease D. Thyroid disease
C. Secondary hyperparathyroidism is a disorder that represents the response of a normally functioning parathyroid gland to chronic hypocalcemia. In most patients, the hypocalcemia is the result of renal disease or vitamin D deficiency. Vitamin D deficiency decreases intestinal calcium absorption, resulting in hypocalcemia. The hypocalcemia resulting from renal disease is more complex. It can result either from the increased serum phosphate level caused by decreased glomerular filtration or from the decreased synthesis of 1,25-dihydroxyvitamin D3 in kidney disease.
What is the normal renal threshold of sodium (measured in millimoles per liter)? A. 80-85 B. 90-110 C. 110-130 D. 135-148
C. Sodium is the principal cation found in the plasma. The normal serum sodium level is 136-145 mmol/L, whereas in urine the sodium concentration ranges between 40 and 220 mmol/day, being dependent on dietary intake. Because sodium is a threshold substance, it is normally excreted in the urine when the serum sodium concentration exceeds 110-130 mmol/L. When serum levels fall below 110 mmol/L, all the sodium in the glomerular filtrate is virtually reabsorbed in the proximal and distal tubules. This reabsorption process is influenced by the hormone aldosterone
Which of the following may be associated with the colloid osmotic pressure (COP) osmometer? A. Utilizes a cooling bath set at -7 degrees Celsius B. Measures total serum osmolality C. Negative pressure on reference (saline) side equivalent to COP of sample D. Measures contribution of electrolytes to osmolality
C. The colloid osmotic pressure (COP) osmometer is composed of a semipermeable membrane that separates two chambers, a mercury manometer, a pressure transducer and a meter. When a serum sample is introduced into the sample chamber, saline solution from the reference chamber moves across the membrane by osmosis. This causes the development of a negative pressure on the saline side that is equivalent to the COP, which represents the amount of protein in the serum sample. COP osmometers measure the serum protein contribution to the total osmolality in terms of millimeters of mercury. COP levls are helpful in monitoring intravenous fluid therapy.
Which of the following conditions is not associated with hyponatremia? A. Addison disease B. Diarrhea C. Diuretic therapy D. Cushing syndrome
D. A decreased serum sodium concentration, or hyponatremia, is associated with a variety of disorders, including (1) Addison disease, which involves the inadequate secretion of aldosterone, resulting in decreased reabsorption of sodium by the renal tubules; (2) diarrhea, which involves the impaired absorption from the gastrointestinal tract of dietary sodium and of sodium from the pancreatic juice, causing an excessive quantity of sodium to be excreted in the feces; (3) diuretic therapy, which causes a loss of water with concurrent loss of electrolytes, including sodium; and (4) renal tubular disease, which involves either the insufficient reabsorption of sodium in the tubules or a defect in the Na+ -H+ tubular exchange mechanism. A diagnosis of Cushing syndrome is incorrect because the disorder is associated with hypernatremia.
A serum ferritin level may not be a useful indicator of iron-deficiency anemia in patients with what type of disorder? A. Chronic infection B. Malignancy C. Viral hepatitis D. All the above
D. In cases of iron-deficiency anemia uncomplicated by other diseases, serum ferritin levls correlate well with the evidence of iron deficiency obtained by marrow examination for stainable iron. This indicates that ferritin is released into the serum in direct proportion to the amount stored in tissues. In iron deficiency, serum ferritin levels fall early in the disease process. However, in certain disorders there is a disproportionate increase in serum ferritin in relation to iron stores. Examples include chronic infections, chronic inflammation, malignancies and liver disease. For individuals who have these chronic disorders or iron deficiency, it is common for their serum ferritin levels to appear normal.
Which of the following is not associated with potassium? A. Has no renal threshold B. Increased serum level in acidosis C. Hemolysis causes false increase in serum levels D. Major anion of intracellular fluid
D. In contrast to sodium, which is the principal plasma cation, potassium is the principal cellular cation. After absorption in the intestinal tract, potassium is partially filtered from the plasma by the kidneys. It is then almost completely reabsorbed from the glomerular filtrate by the proximal tubules and subsequently re-excreted by the distal tubules. Unlike sodium, potassium exhibits no renal threshold, being excreted into the urine even in K+-depleted states. In acidotic states, as in renal tubular acidosis in which the exchange of Na+ for H+ is impaired, the resulting retention of potassium causes an elevation in serum K+ levels. Hemolysis must be avoided in blood specimens that are to be used for K+ analysis because erythrocytes contain a potassium concentration 23 times greater than serum K+ levels. If the red blood cells are hemolyzed, a significant increase in serum K+ will result.
Which of the following is an effect of increased parathyroid hormone secretion? A. Decreased blood calcium levels B. Increased renal absorption of phosphate C. Decreased bone resorption D. Increased intestinal absorption of calcium
D. PTH has physiological actions on bone, kidney and intestine. Its overall effect is to raise serum ionized calcium levels and lower serum phosphorus levels. Its actions on various organs are the result of a combination of both direct and indirect effects. In bones, PTH directly acts to increase bone resorption, thereby increasing both calcium and phosphorus in the blood. In the kidneys, PTH directly acts on the renal tubules to decrease in blood phosphorus levels. In the intestines, PTH acts to increase absorption of calcium by its action in increasing 1,25-dihydroxyvitamin D3 synthesis in the kidneys, which in turn stimulates intestinal absorption of calcium
Which of the following does not have an effect on plasma calcium levels? A. Parathyroid hormone B. Vitamin D C. Calcitonin D. Aldosterone
D. PTH, calcitonin, vitamin D, plasma proteins and plasma phosphates are factors that influence plasma calcium levels. PTH is a hormone important in maintaining plasma calcium levels. It mobilizes calcium from bones. It increases the synthesis of one of the vitamin D derivatives, thereby causing an increase in bone resorption and intestinal absorption of calcium. When normal calcium levels are restored, PTH secretion is cut off (negative-feedback mechanism). Calcitonin (thyrocalcitonin) is a hormone secreted by the thyroid gland in response to elevated levels of plasma calcium. It acts by inhibiting bone resorption of calcium, thereby preventing significant variations in plasma calcium concentrations. Hydroxylation of vitamin D gives a derivative that will increase the intestinal absorption of calcium and phosphates.
Primary aldosteronism results from a tumor of the adrenal cortex. How would the extracellular fluid be affected? A. Normal sodium, decreased potassium levels B. Decreased sodium, decreased potassium levels C. Decreased sodium, increased potassium levels D. Increased sodium, decreased potassium levels
D. Primary aldosteronism is characterized by the hypersecretion of aldosterone, a mineralocorticoid, by the zona glomerulosa cells of the adrenal cortex. Excessive secretion of aldosterone will increase renal tubular reabsorption of sodium, resulting in a decrease in the loss of sodium in the urine. The net result of this mechanism is increased sodium in the extracellular fluid. Additionally, there will be increased renal excretion of potassium, causing a decrease of potassium in the extracellular fluid.
Which of the following describes the basis for the freezing point osmometer? A. The freezing point depression is directly proportional to the amount of solvent present B. The freezing point depression varies as the logarithm of the concentration of solute C. The freezing point is raised by an amount that is inversely proportional to the concentration of dissolved particles in the solution D. The freezing point is lowered by an amount that is directly proportional to the concentration of dissolved particles in the solution
D. The freezing point of an aqueous solution is lowered 1.86 degrees Celsius for every osmole of dissolved particles per kilogram of water. These particles may be ions (e.g. Na+ and Cl-), or undissociated molecules such as glucose. The freezing point osmometer is an instrument designed to measure the freezing point of solutions. It uses a thermistor that is capable of measuring very small changes in temperature.
Measuring the tubular reabsorption of phosphate is useful in diagnosing diseases that affect which of the following organs? A. Liver B. Adrenal gland C. Thyroid gland D. Parathyroid gland
D. The renal tubular reabsorption of phosphate is controlled by the action of parathyroid hormone (PTH) on the kidney. Increased PTH secretion from any cause will lead to a decreased tubular reabsorption of phosphate (increased urine phosphate and decreased serum phosphate). The test is useful in distinguishing serum hypercalcemia that is a result of excess PTH production by the parathyroid glands from hypercalcemia due to other causes (e.g., bone disease).
Given the following information, calculate the plasma osmolality in milliosmoles per kilogram: sodium -> 142 mmol/L; glucose ->130 mg/dL; urea nitrogen ->18 mg/dL A. 290 B. 291 C. 295 D. 298
D. When the osmolality has been both measured in the laboratory and calculated, the osmolal gap may then be determined by subtracting the calculated osmolality from the measured. Plasma osmolality may be calculated when the plasma sodium, glucose and urea nitrogen values are known. The equation for calculating osmolality expresses Na+, glucose and urea nitrogen in mmol/L (SI units). To convert glucose and urea nitrogen from mg/dL to mmol/L, the conversion factors 0.056 and 0.36 are used, respectively. For sodium, the factor 2 is used to count the cation (sodium) once and its corresponding anion once. Because glucose and urea nitrogen are undissociated molecules, they are each counted once. Use the follwoing equation: Calculated osmolality (mOsm/kg) = 2.0 Na+ (mmol/L) + Glucose (mmol/L) + Urea nitrogen (mmol/L) = 2.0 (142 mmol/L) + (0.056 x 130 mg/dL) + (0.36 X 18 mg/dL) 284+7.3+6.5=298 mOsm/kg