Clinical Chemistry MCQ

Blood urea nitrogen (BUN) and creatinine are con sidered markers of kidney function. Renal failure can cause elevations in BUN and other prerenal conditions such as dehydration or a high-protein diet that raise levels of BUN without affecting creatinine levels.

Whole blood glucose levels are 12% to 15% lower than plasma glucose levels. Therefore a plasma glucose level of 100 mg/dL would roughly correspond to a whole blood level of 85 mg/dL.

The anion gap, especially when elevated, is useful in diagnosing the type of metabolic acidosis and in indicating if a mixed disorder exists.

The VLDL fraction is primarily composed of triglycerides and lesser amounts of cholesterol and phospholipids. Protein components of VLDL are mostly apolipoprotein B-100 and apolipoprotein C. VLDL migrates electrophoretically in the prebeta region.

Lipase and amylase are both markers of pancreatic function; however, lipase is considered more specific because it remains elevated longer in acute pancreatitis.

Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are the most sensitive
markers of hepatocellular damage.gamma-Glutamyltransferase (GGT) and alkaline phosphatase (ALP) are markers of hepatobiliary damage. Ammonia, although it may be seen in severe cases of liver damage, is not a sensitive marker of hepatocellular damage.

The serum proteins are divided into five principal fractions based on their electrophoretic mobilities. The five fractions are albumin, alpharglobulin, alpha2-globulin, beta-globulin, and gamma-globulin. Albumin constitutes the largest individual fraction of the serum proteins. The reference concentration of albumin in serum ranges between 3.5 and 5.0 g/dL, and the total globulin concentration is between 2.3 and 3.5 g/dL.

Radionuclides are quantified by measuring the amount of energy that they emit. This can be in the form of alpha emission (4/2)He^2+, beta emission(electrons ejected from the nucleus of a radioisotope during radioactive decay), or gamma emission (electromagnetic radiation emitted during radioactive decay). Beta and gamma emissions can be detected by scintillation counters. The sensing element of a scintillation counter is a fluor, a substance capable of converting radiation energy to light energy.

All enzymatic methods to measure triglycerides, regardless of the enzyme used, begin with the conversion of triglycerides to glycerol and fatty acids in the presence of the enzyme lipase, followed by the conversion of glycerol to glycerol-3-phosphate in the presence of the enzyme glycerol kinase.

Amylase is an enzyme that can degrade complex carbohydrate molecules such as starch.

Estrogens are a group of steroids responsible for the development of female sex organs. The estrogens include estrone, estradiol, and estriol. Estradiol is the predominant form of estrogen.

The free thyroxine (FT4) index is calculated as follows:

Both total cholesterol and HDL cholesterol are independent measurable indicators of risk of coronary heart disease (CHD). By relating total and HDL cholesterol in a mathematical way, physicians can obtain valuable additional information in predicting risk for CHD. Risk of CHD can be quantified by the ratio of total cholesterol to HDL cholesterol along the following lines:
Ratio Risk CHD
3.43 half average
4.97 average
9.55 two times average
24.39 three times average
Thus this patient shows approximately twice the average risk for CHD. Risk ratios for CHD can easily be calculated by instrument and/or laboratory computers given the total and HDL cholesterol values. Reports indicating level of risk based on these results can be programmed by the laboratory and/or manufacturer.

Osmolality and specific gravity both measure the solute concentration of a solution. Specific gravity measures solute concentration as the solute’s density, which is subject to interference from large molecules such as glucose and proteins. Osmolality, on the other hand, measures solute concentration as the number of molecules present by measuring the number of molecules per kilogram of water.

Apo A-I is the predominant apoprotein associated with the high-density lipoprotein (HDL) molecule, activates (lecithin cholesterol acyltransferase[LCAT]), and is associated with reverse cholesterol transport. As a result, it is protective against coronary artery disease.

Room air contains a pO2 of 150 mm Hg and a pCO2 of approximately zero. When a blood gas
sample is received in the laboratory uncapped or if it contains air bubbles, gas equilibration between the air and the blood will occur, causing a decreased pCO2, increased pO2, and increased pH. Blood gas samples from a patient without oxygen supplementation with a pO2 greater than 110 mm Hg should be investigated for air contamination.

The ion-exchange electrode is a type of potentiometric, ion-selective electrode that consists of a liquid ion-exchange membrane that is made of an inert solvent and an ion-selective neutral earner material. A collodion membrane may be used to separate the membrane solution from the sample solution being analyzed. Because of its ability to bind K+, the antibiotic valinomycin is used as the neutral carrier for the K+ selective membrane. The antibiotics non actin and monactin are used in combination as the neutral carrier for the NH^-selective membrane. A special formulation is used to make a selective glass membrane for the measurement of sodium.

The concentration of total protein in cerebrospinal fluid (CSF) is 15^5 mg/dL. Such a low level of protein requires a method with sufficient sensitivity such as Coomassie brilliant blue. Turbidimetric methods can also be used to quantify protein in CSF. Neither biuret nor Ponceau S has the sensitivity needed, and bromcresol green measures only albumin and does not react with the globulins.

The reagent blank contains the same reagents as those used for assaying the specimen. By adjusting the spectrophotometer to IOQ%T (or 0 absorbance) with the reagent blank, the instrument automatically subtracts the color contributed by the reagents from each succeeding reading of specimens, controls, and standards. This technique is used both in manual procedures and automated instruments. Because the reagent blank does not contain sample, there is no correction for interfering chromogens or lipemia.

In renal disease, glomerular or tubular malfunction results in proteinuria. In early stages of glomerular dysfunction, small quantities of albumin will appear in the urine. Because the concentration is so low, urine dipstick assays are unable to detect the presence of such a small quantity of albumin; hence the term "microalbuminuria." Annual testing of diabetic individuals for microalbuminuria is recommended, because identification of these low levels of albumin that precede nephropathy would allow for clinical intervention to control blood glucose levels and blood pressure. The reference interval for urinary albumin is less than 30 mg/day. Microalbuminuria may be quantified using immunonephelometry and enzyme immunoassay.

As part of a good quality assurance program, a laboratory should perform function verification, performance verification, and preventive maintenance for all instrument systems. Function verification is the monitoring of specific instrument functions and the correcting of these functions when necessary to assure reliable operation. Function verification includes monitoring temperature, setting electronic parameters, calibrating instruments, and analyzing quality control data. It is important that performance of these activities be properly documented.

In a state of acidosis there is an increased amount of hydrogen ions that combine with albumin. Because H ions are combining with albumin, less albumin is available to bind to calcium; therefore ionized calcium levels increase.

Insulin lowers glucose levels by increasing the uptake of glucose into the cell and through increased glucose metabolism.

PCO2 is an acidic gas controlled by the lungs (respiratory system) and when in increased concentrations will cause respiratory-driven acidosis.

A number of immunochemical assays can be used to quantify the apolipoproteins. Some of the techniques that can be used include immunonephelometric assay, enzyme-linked immunosorbent assay (ELISA), and immunotur bidimetric assay. Commercial kits are available for the quantification of Apo A-I and Apo B-100. Measuring the apolipoproteins can be of use in assessing increased risk for coronary heart disease.

Adequate specificity is generally obtained when using the urease/glutamate dehydrogenase method. Because urease hydrolyzes urea to ammonia and water, a positive interference from endogenous ammonia will occur with elevated blood levels of ammonia. Such interference may occur from use of aged blood specimens and in certain metabolic diseases.

The majority of serum enzymes that are of interest clinically are of intracellular origin. These enzymes function intracellularly, with only small amounts found in serum as a result of normal cellular turnover. Increased serum levels are due to tissue damage and necrosis, where the cells disintegrate and leak their contents into the blood. Thus, elevated serum levels of intracellular enzymes are used diagnostically to assess tissue damage

The kidney is responsible for acid-base balance through the removal of H ions via (1) the reaction of the hydrogen ions with bicarbonate, (2) the reaction of hydrogen ions with filtered buffers such as disodium salt, (3) reaction with ammonia, and (4) excretion of the free hydrogen ions.

The clinical usefulness of AST and LDH has been replaced primarily by cardiac troponin and to a lesser degree by myoglobin, whereas CK isoenzymes continue to play a role. Although myoglobin will increase above the upper reference interval in 1-3 hours following AMI, it is not tissue specific for cardiac muscle and its application has found limited usefulness. Myoglobin will also be increased following skeletal muscle trauma. Troponin I and troponin T have proven to be useful markers, because each has a cardiac-specific isoform, cTnl and cTnT. cTnl appears to be more specific for cardiac muscle, because it has not been identified in regenerating or diseased skeletal muscle, whereas cTnT is made in small amounts by skeletal muscle. Total CK is elevated in AMI and takes 4-6 hours to rise above the upper reference interval.

In the catabolic process of hemoglobin degradation, the alpha-carbon methene bridge of the tetrapyrrole ring structure of heme opens oxidatively to form verdohemoglobin. Verdohemoglobin is a complex composed of biliverdin, iron, and the protein globin. This complex then undergoes degradation in which iron is removed and returned to the body iron stores, the globin portion is returned to the amino acid pool, and the biliverdin undergoes reduction to form bilirubin. It is biliverdin, therefore, that is the immediate precursor of bilirubin formation. Mesobilirubinogen and urobilinogen represent intestinal breakdown products of bilirubin catabolism.