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Clinical Chemistry (Electrolytes)
Terms in this set (50)
What are electrolytes?
Substances that carry an electric current when dissolved in water. Anions are negatively charged and cations are
What are the major electrolytes?
Sodium, potassium, chloride, and bicarbonate
What is the major extracellular cation?
What is the major intracellular cation?
What is the major intracellular anion?
What is the major extracellular anion?
Which hormone regulates the concentration of sodium?
Why should standards for sodium be stored in plastic containers instead of glass?
Sodium can leech from glass containers and cause falsely elevated values.
How are sodium and potassium usually measured?
By ion selective electrodes (ISE).
What is the approximate ratio of potassium between RBCs and serum?
20: 1. This explains why hemolysis must be avoided when measuring potassium.
What are several factors that can cause artifactual (false) elevations of potassium?
Fist clenching, prolonged tourniquet time, contamination with K2EDTA or IV fluid, hemolysis, thrombocytosis,
Why is potassium slightly higher in serum than in plasma?
Because potassium is released from platelets during clotting.
Before reporting an elevated potassium, what should be checked?
The specimen should be checked for hemolysis or excessive delay in separating the serum/plasma from the RBCs. Either could be responsible for a false elevation.
What clinical condition results from very high or very low potassium levels?
What is chloride's role in the body?
It maintains hydration, osmotic pressure, and electrolyte balance.
What is pilocarpine?
A topical drug used in the sweat test to stimulate sweat glands.
In iontophoresis, an electric current delivers
pilocarpine nitrate to the sweat glands on the forarm or thigh. Sweat is collected and chloride is measured. A source of error is failure to collect an adequate volume of sweat. The sweat test should be performed at a CF Foundation accredited care center.
What happens to CO2 if the sample is exposed to air?
What does anion gap measure?
Unmeasured anions. The anion gap is increased with renal failure, diabetic acidosis, lactic acidosis, and the presence of drugs or toxins.
What is the most abundant mineral. in the body?
Which anticoagulants cause a false decrease in calcium?
EDTA, citrate, and oxalate. All prevent coagulation by binding Ca* . Heparin is the only acceptable anticoagulant
for calcium determinations.
Which form of calcium is physiologically active?
Why is pH an important consideration in ionized calcium determinations?
As pH decreases (acidosis), calcium dissociates from its complexed forms, increasing the amount of free ionized
calcium in the serum
What is the commonly used method for total calcium?
A dye-binding reaction with ortho-cresolphthalein complexone (CPC) or arsenazo III. Atomic absorption is the
reference method but rarely used in clinical labs.
How is ionized calcium measured?
By ion selective electrodes.
What substances regulate calcium levels?
PTH, calcitonin, and vitamin D.
What is tetany?
Muscle spasms, cramps, and irritability due to decreased calcium or magnesiu
What is the most common cause of hypercalcemia?
How does hyperparathyroidism affect the level of serum phosphorus?
Phosphorus is decreased.
What happens to calcium when phosphorus is increased?
It decreases. There is a reciprocal relationship between calcium and phosphorus.
A hospitalized patient exhibits signs of tetany but her ionized calcium is normal. What other analyte should
Magnesium. Low levels of magnesium also cause tetany.
How does the reference range for phosphorus in growing children compare to that of adults?
It is higher in children.
What must be done to urine prior to performing a urine phosphorus analysis?
It must be acidified to pH 6 to prevent precipitation of phosphates.
How does hemolysis affect iron level?
Because of the high concentration of iron in hemoglobin, even minimal hemolysis will give falsely elevated results.
To minimize this effect, serum/plasma should be separated from RBCs within one hour of collection and even slightly hemolyzed specimens should not be analyzed.
How are iron levels affected by the time of day when the specimen is drawn?
Iron shows a marked diurnal variation. Levels are approximately 30% higher in the morning.
Which protein transports iron?
Transferrin. It is normally 20-55% saturated with iron.
Where is most of the iron in the body?
Name 2 storage forms of iron.
Ferritin is the primary storage form. It is present in most cells and is a readily mobilized form of storage iron. A
small amount of iron is also stored as hemosiderin
How are the iron and total iron binding capacity (TIBC) affected in iron deficiency anemia?
Serum iron is decreased and TIBC is increased. TIBC is an indirect measurement of transferrin. TIBC is
infrequently performed since the development of improved transferrin assays.
What is the most sensitive test for detection of iron deficiency anemia?
Serum ferritin. A decreased serum ferritin is almost always indicative of iron deficiency anemia.
What is hereditary hemochromatosis and what tests are used for diagnosis?
Hereditary hemochromatosis is the most common of the iron overload diseases. It causes the body to absorb and
store too much iron. The iron panel consists of serum iron (SI), ferritin, total iron binding capacity (TIBC) and
transferrin saturation (TS, also known as percent saturation). TS determines how much iron is bound to transferrin, the protein that carries iron in the blood. After a 12-hour fast, the TIBC and SI are measured and the TS is
calculated (Sl/TIBC = TS). The serum ferritin test shows the level of iron in the liver.
What test results are typical in hereditary hemochromatosis?
Serum iron, ferritin, and transferrin saturation are all increased. Total iron binding capacity is decreased.
What is lactate?
Lactic acid, an intermediary in carbohydrate metabolism. There are 2 types of lactic acidosis:
• Hypoxic, due to decreased oxygen delivery to the tissues
• Metabolic, associated with disease, drugs/toxins, and inborn errors of metabolism
The mortality rate for lactic acidosis is greater than 60%.
Name a reagent used to measure lactate.
Lactate dehydrogenase (LD). Lactate is oxidized to pyruvate by LD in the presence ofNAD+. The NADH formed is measured at 340 nm.
What happens to lactate in the blood following collection?
It increases due to glycolysis.
What is a colligative property?
One that depends on the number of solute particles, regardless of size or molecular weight. The colligative
properties are osmotic pressure, vapor pressure, boiling point, and freezing point.
How is osmolality usually measured in the clinical lab?
By freezing point depression.
What does the urine to serum osmolality ratio indicate?
The degree to which the kidneys concentrate the glomerular filtrate. The normal urine:serum ratio is
1:1- 3: 1.
Which substance contributes most to serum osmolality?
Sodium accounts for almost half.
What are several clinical conditions that result in an increased serum osmolality?
Dehydration, uremia, diabetes mellitus, alcohol intoxication, salicylate intoxication, and excessive electrolyte IVs.
What is osmolal gap?
The difference between measured osmolality and calculated osmolality. It is used to diagnose poisonings and to estimate blood alcohol levels. The reference range is 0-10 mOsm/kg. Higher levels indicate an abnormal
concentration of an unmeasured substance such as isopropanol, methanol, ethylene glycol, or acetone.
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