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C HAP TE R 11 / Laboratory Tests Using Blood 263

higher because the number of nephrons tends to decrease in the
aging process. The BUN may be higher in hospitalized patients
Table 11-5 ■ COMPARISON BETWEEN SPECIFIC HBA 1C
LEVELS AND SPECIFIC MEAN PLASMA GLUCOSE LEVELS because of their increased catabolic state. 97
Mean Plasma Glucose
Uric Acid
HbA 1C (%) mg/dL mmol/L
Uric acid is synthesized in the liver and intestinal mucosa and is
6 135 7.5 the end product of purine metabolism. Uric acid may be in-
7 170 9.5
8 205 11.5 creased in a variety of conditions but the most common cause in
9 240 13.5 an increase in blood levels is gout. Levels are monitored during
10 275 15.5 the diagnosis and treatment of gout. Levels may also be increased
11 310 17.5 in patients with atherosclerosis, hypertension, and elevated
12 345 19.5
triglycerides. Severe renal disease results in a high level of serum
uric acid because excretion is reduced. Levels are very labile and
From Rohlfing, C. L., Wiedmeyer, H. M., Little, R. R., et al. (2002). Defining the rela-
show day-to-day and seasonal variation within the same person.
tionship between plasma glucose and HbA 1C : Analysis of glucose profiles and HbA 1C
in the diabetes control and complications trial. Diabetes Care, 25, 276. Levels are also increased by emotional stress, total fasting, and in-
creased body weight. Large doses of salicylates may interfere with
accurate test results. 97
cells and is released during tissue injury. LDH is widely distrib-
uted in the body. It can be found in skeletal muscle, RBCs, kid- BLOOD CULTURES
neys, liver, pancreas, lungs, and brain. Because of its presence in
multiple organs throughout the body, evaluation of LDH is used Blood cultures are indicated when a fever of unknown origin is
to help establish many diagnoses. LDH has become obsolete in its present. Blood cultures aid in identifying specific bacterial organ-
use as a diagnostic aid in MI due to the more specific and sensi- isms in the blood (bacteremia), and when combined with antibi-
tive troponin markers. Drugs that may cause an elevated LDH in- otic sensitivity tests, can provide information to clinicians about
clude clofibrate, codeine, meperidine, morphine, procainamide, which antibiotic works best against that particular species of bac-
and lipid-lowering agents such as HMG-CoA reductase inhibitors teria. Policies differ with regard to the number and timing of cul-
(statins) and nicotinic acid. 2,3 The normal reference range for tures considered adequate for diagnosis; the policy and procedure
LDH is listed in Table 11-4.
of the institution should be followed. Regardless of the number of
cultures recommended and the timing between them, collection
Protein of blood cultures requires meticulous technique to protect the
specimen from contamination. Sampling should be done while
Total protein measurement includes albumin (53%) and globulin the patient’s temperature is still elevated and before treatment
(15% , 12% , and 20% ). These protein components can be with antibiotics. Both Beutz et al. 98 and Martinez et al. 99 deter-
quantified with the use of protein electrophoresis. Albumin (4 to mined that, although blood cultures may be obtained from a cen-
5.5 g/dL) contributes to the balance of osmotic pressure between tral IV catheter, they are considered less sensitive than through
blood and tissues. Globulins (2 to 3 g/dL) influence osmotic pres- venipuncture but offer an excellent negative predictive value in di-
sure and include the immunoglobulins (antibodies). Because al- agnosing bacteremia. With either method of blood draw, the blood
bumin is produced in the liver, a low serum albumin level is seen is placed into a specialized culture media. Preliminary results
in liver disease. Low serum albumin also reflects poor nutritional should be available within 24 hours, but final results may not be
status, and the finding should prompt a complete nutritional as- available for a week or more. 2,3
sessment. The half-life of albumin is 18 days. If albumin is re-
duced, edema results because albumin accounts for 90% of the
serum colloid osmotic pressure. Albumin is reduced in heart fail-
ure because of hypervolemic dilution. The - and -globulins SERUM CONCENTRATION
tend to decrease with abnormal liver function. The -globulins, OF SELECTED DRUGS
the body’s antibodies, increase with chronic disease. 2,3
Serum levels of cardiac drugs may be obtained for multiple rea-
Urea Nitrogen sons including determining the effectiveness of drug therapy, es-
pecially for drugs with narrow therapeutic ranges or with wide
Urea nitrogen is the end product of protein metabolism. It is pro- variabilities in metabolism between patients, and confirming
duced by the liver and excreted by the kidneys. BUN is used with cause of organ toxicity. Usual ranges of therapeutic and toxic
creatinine to evaluate renal function. Increases in BUN are re- serum concentrations of selected cardiac drugs are given in Table
ferred to as azotemia. Prerenal azotemia occurs whenever a disease 11-6.
or condition affects urea nitrogen before the kidneys are actually The serum concentrations must always be interpreted in the
damaged or diseased including congestive heart failure, salt and context of the clinical data. For example, digitalis intoxication
water depletion from vomiting, diarrhea, diuresis, sweating, or may occur within the usual range of therapeutic serum concen-
2
shock. Postrenal azotemia is the result of any condition that af- trations if the patient has hypokalemia, hypercalcemia, hypo-
fects BUN after it has cleared the kidneys, such as in ureteral and magnesemia, acid–base imbalances, increased adrenergic tone,
urethral obstruction. BUN levels in older adults may be slightly hypothyroidism, hypoxemia, or myocardial ischemia. 100