NCLEX-RN: Laboratory Tests

Laboratory Tests: Thyroid Function Tests

Focus topic: Laboratory Tests

Laboratory Tests: Radioactive Iodine Uptake (Radioiodine 131I)

Focus topic: Laboratory Tests

A. Normal values—5–35% in 24 hours (recently lowered values in United States due to increased ingestion of iodine).

  •  Elevated values indicate hyperthyroidism, thyrotoxicosis, hypofunctioning goiter, iodine lack, excessive hormonal losses.
  •  Depressed values indicate low T4, antithyroid drugs, thyroiditis, myxedema, or hypothyroidism.

B. Purpose—measures the absorption of the iodine isotope to determine how the thyroid gland is functioning.

C. Principles.

  •  The use of 123I rather than 131I is now preferred because of its lower radiation hazard. (123I can be used on pregnant women; 131I is contraindicated.)
  • The amount of radioactivity is measured at 2, 6, and 24 hours after ingestion of the capsule.
  •  131I (as does 123I) evaluates the storage of iodine and gives a distribution pattern.
Laboratory tests

Laboratory Tests: Thyroid-Stimulating Hormone Ultra sensitive Assay

Focus topic: Laboratory Tests

A. Normal values were 0.5–5.0 μU/mL—new, narrower thyroid-stimulating hormone (TSH) normal range of 0.3–3.0 μU/mL is a more accurate level and is recommended to become the standard of practice for therapeutic management.

  •  Increased values, more than 20 μU/mL indicates hyperthyroidism, Addison’s disease, goiter, and toxicity from certain drugs.
  • 2. Decreased values: first-degree (secondary) hypothyroidism is less than 0.3 μU/mL, and second- to third-degree hypothyroidism is less than 0.1 μU/mL.

B. Test is an ultra sensitive indicator that has mostly replaced all other thyroid tests.

  •  If assay is normal, no other test is indicated.
  •  If test is abnormal, it should be validated by a T4 assay.

Laboratory Tests: T3 and T4 Resin Uptake Tests

Focus topic: Laboratory Tests

A. Normal values.

  •  T4—3.8–11.4%.
  •  T3—25–35%.
  •  T4.
    a. Elevated—hyperthyroidism, early hepatitis, exogenous T4.
    b. Decreased—hypothyroidism, abnormal binding, exogenous T4.
  • T3.
    a. Elevated—hyperthyroidism, T3 toxicosis.
    b. Decreased—advancing age.

B. Purpose—both of these in vitro tests are used as screening tests for diagnosis in thyroid disorders. T4 is 90% accurate in diagnosing hyperthyroidism and hypothyroidism.

C. Decreased T4 and normal or elevated TSH level can indicate thyroid disorder; decreased T4 and decreased TSH level can indicate pituitary disorder.

D. Principles.

  •  Levels of T3 and T4 in the blood regulate TSH.
  • These levels change according to a balancing system of negative feedback.
  •  Venous blood sample is obtained to directly measure concentration of unsaturated thyroxine-binding globulin in the serum.
  • Thyroid function tests should be interpreted according to the clinical situation.

Laboratory Tests: TSH

Focus topic: Laboratory Tests

A. Normal values 0–6 μU/mL or < 10 μU/mL (may vary with laboratory).

  •  Increased values indicate primary hypothyroidism.
  •  Decreased values indicate Hashimoto’s thyroiditis, hyperthyroidism, large doses of glucocorticoids, secondary hypothyroidism.

B. Purpose—differentiates primary from secondary hypothyroidism and assesses level of thyroid gland activity.

C. Principles.

  • Administration of IM TSH (thyrotropin) measures the responsiveness of the thyroid gland.
  • Blood samples are obtained at intervals.

Laboratory Tests: Blood Glucose Studies

Focus topic: Laboratory Tests

Laboratory Tests: Fasting Plasma Glucose

Focus topic: Laboratory Tests

A. Normal fasting glucose is 70 to 100 mg/100 mL; indicates good metabolic control.

B. > 125 mg/dL can signify diabetes.

  •  This number is based on the most recent guidelines that lowered the threshold for diabetes.
  •  Fasting is defined as no calorie intake for 8 hours.

C. Fasting plasma glucose (FPG) is used to diagnose hypoglycemia, confirm a diagnosis of prediabetic state, confirm diabetes mellitus, or monitor blood glucose levels.

Laboratory Tests: Random (Casual) Plasma Glucose Levels

Focus topic: Laboratory Tests

A. Levels of more than 200 mg/dL on more than one occasion are diagnostic of diabetes.

B. Casual is any time of day without regard to when the last meal was eaten.

Laboratory Tests: Glucose Tolerance Test

Focus topic: Laboratory Tests

A. Normal values are between 70 and 105 mg fasting blood glucose and no sugar in the urine.

  •  Greater than 140 mg/dL fasting and 200 mg/dL 2 hours postprandial are diagnostic of diabetes.
  •  The oral glucose tolerance test and IV glucose are no longer recommended for routine clinical use.

B. Purpose—primary aim is to diagnose or rule out diabetes, but also important for unexplained hypoglycemia and malabsorption syndrome.

C. Principles.

  •  This test determines rate of removal of a concentrated dose of glucose from the bloodstream.
  •  Test is indicated when there is sugar in the urine or when fasting blood sugar is elevated.
  •  This is a timed test done in the morning after fasting for at least 12 hours. Blood and urine samples are taken at intervals up to 3 hours.
  • This test is contraindicated for recent surgical clients or clients with history of myocardial infarctions.

Laboratory Tests: Immunodiagnostic Tests

Focus topic: Laboratory Tests

Laboratory Tests: HIV-1 Antibody Test

Focus topic: Laboratory Tests

A. The ELISA (enzyme-linked immunosorbent assay) test was developed to screen donor blood on a national scale.

  •  This test does not test for acquired immune deficiency syndrome (AIDS), but rather antibodies to the human immunodeficiency virus (HIV).
  •  Once exposed to a virus, it takes the body time to produce antibodies. A person may already be infected and if the body has not yet produced antibodies, the ELISA test will be negative.
  •  The test is not perfect because it may produce a false positive or false negative.
  •  When performed at least 12 weeks after infection, the test has a 99.5% sensitivity and will show a positive result.

B. All positive results must be retested.

C. The Western blot test is given for final confirmation; used to confirm seropositive blood as identified by ELISA.

D. The indirect immunofluorescence assay (IFA) is being used by some physicians rather than the Western blot to confirm positive HIV. This test is rapid and easy to complete.

Laboratory Tests: Coombs’ Test

Focus topic: Laboratory Tests

A. Normal values negative.

B. Purpose test to discover presence of antibodies present in Rh-negative mother’s blood.

  • Test also will confirm diagnosis of hemolytic disease in the newborn.
  •  Titration determines extent to which antibodies are present.

C. Types of test for Rh incompatibility.

  •  Indirect Coombs’ mother’s blood reveals antibodies as result of previous transfusion or pregnancy.
  •  Direct Coombs tests newborn’s cord blood: determines presence of maternal antibodies attached to baby’s cells.

Laboratory Tests: Venereal Disease Research Laboratory Test

Focus topic: Laboratory Tests

A. Normal values serum is nonreactive.

B. Purpose to screen for primary or secondary syphilis and for diagnosis.

C. Differential diagnosis.

  • Biological false-positive tests may occur with hepatitis, mononucleosis, leprosy, malaria, rheumatoid arthritis, lupus erythematosus.
  •  A nonreactive result does not rule out syphilis, as it takes up to 4 weeks after infection to cause an immunologic response.

D. The rapid plasma reagin circle card test (RPR-CT) is also used to screen for diabetes.

E. The fluorescent treponemal antibody absorption test (FTA-ABS) is used to verify the screening test and determine that it was not a false positive.

Laboratory Tests: Epstein-Barr Virus Antibodies

Focus topic: Laboratory Tests

A. Normal values negative (antibodies appear within first 3 weeks, then decline rapidly).

B. Purpose to diagnose infectious mononucleosis or to determine the antibody status of EBV-infected people.

C. Test serum is tested for heterophile antibodies (Monospot test).

D. Differential diagnosis.

  • Positive results may occur with infectious mononucleosis, hepatitis A and B, cancer of the pancreas.
  •  A negative Monospot does not always rule out acute or past EBV.

Laboratory Tests: Serologic Tests for Hepatitis A and B

Focus topic: Laboratory Tests

A. Normal values negative for hepatitis A, B, non-A, non-B, and D.

B. Purpose serologic tests diagnose and differentiate different forms of hepatitis and detect presence in client’s or donor’s blood.

C. Test variations.

  •  Hepatitis A (HAV).
    a. Anti-HAV immunoglobulin M (IgM) presence confirms recent infection of hepatitis A detectable for 3 to 12 weeks.
    b. Anti-HAV immunoglobulin G (IgG) indicates previous exposure to HAV, recovery, and immunity. Appears after acute 2. Hepatitis B (HBV).
    a. Hepatitis B surface antigen appears in 27 to 41 days and is the earliest indicator of HBV.
    b. Antibody to hepatitis B surface antigen indicates clinical recovery with subsequent immunity.
  •  Hepatitis C (non-A, non-B) has no serologic or laboratory test to establish diagnosis usually made by excluding other causes of hepatitis.infection and is detectable for life.
  •  Hepatitis D or delta is associated with hepatitis B and depends on HBV for replication. It is found in the serum 7 to 14 days during acute infection.
  • Hepatitis E virus fecal oral route; similar to HAV.
  •  Hepatitis G also known as GB virus C.

Laboratory Tests: Rubella (German Measles) Viral Serologic Test

Focus topic: Laboratory Tests

A. Normal values.

  •  Negative titer of less than 1.8 or 1.10 (depending on test) no antibody detected, therefore not immune.
  •  Positive titer of more than 1.10 antibody detected, therefore immune.

B. Purpose exposure to rubella is important to detect because exposure to this virus if a woman is in the first trimester of pregnancy may result in congenital abnormalities, abortion, or stillbirth.

Laboratory Tests: Papanicolaou Smear

Focus topic: Laboratory Tests

A. Diagnosis to identify preinvasive and invasive cervical cancer.

B. Vaginal secretions and secretions from posterior fornix are swabbed and smeared on a glass slide.

C. Pathological classifications (early cellular changes may be detected before disease becomes clinically observable).

  •  Class I—no abnormal or atypical cells present.
  •  Class II—atypical or abnormal cells present but no malignancy found; repeat Papanicolaou (Pap) smear and follow up if necessary.
  •  Class III—cytology, suggestive of malignancy; additional procedures indicated (biopsy, dilation and curettage [D&C]).
  •  Class IV—cytology, strongly suggestive of malignancy; additional procedures indicated (biopsy, D&C).
  •  Class V—cytology results conclusive of malignancy.

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