LAB DX EXAM 1 REVIEW



LAB DX EXAM 1 REVIEW

Know normal values:

|WBC |(5-10) |

|RBC |(4.2-6.0) |

|HgB |(12-15.5) F |

| |(12.5-17.5) M |

|HCT |(37-45) F |

| |(40-50) M |

|MCV |(80-99) |

|Fasting Glucose |(80-115)mg/dl |

|BUN |(7-27)mg/dl |

|Creatinine |(0.5-1.5)mg/dl |

|Sodium |(135-148)meq/l |

|Potassium |(3.4-5.3)meq/l |

|Chloride |(95-108)meq/l |

|Calcium |(8.1-10.7)mg/dl |

|Albumin |(3.3-5.5)g/dl |

|Uric Acid |(2.5-7.7)mg/dl |

|Magnesium |(1.8-2.6)mg/dl |

|Phosphate |(2.7-4.5)mg/dl |

|Bilirubin (total) |(0.2-1.2)mg/dl |

|Cholesterol (HDL) |(29-72)mg/dl |

|Cholesterol (LDL) |‹130 mg/dl |

List and give examples of uses or reasons for ordering lab tests:

▪ Laboratory tests have greater specificity and sensitivity than the physical examination for many disorders

▪ Lab Tests Should be utilized as an Aid to Clinical Judgment

UTILITY OF LABORATORY TESTS:

1. Conformation of Clinical Impression

Ex: fasting glucose for diabetes mellitus

2. Reduction of Differential Diagnosis

Ex: serum amylase (appendicitis, cholecystitis, pancreatitis)

3. Disease Exclusion

Ex: normal glucose for hypoglycemia

4. Indication of Specific Therapy

Ex: Iron, Folate, B12, etc.

5. Monitor Therapy Effectiveness

Ex: serum albumin/protein for malnutrition

6. Disclose Occult Disease

Ex: stool occult blood for colon cancer

7. Determination of Baseline Values

Ex: patient’s own normal values, severity of disease

8. Staging of Disease

Ex: prodromal, acute, chronic, convalescence

9. Defensive Chiropracitc

Ex: malpractice, negligence, patient responsibility

Define, describe and give example of:

Accuracy of a lab test:

▪ Accuracy of a laboratory result refers to how close the result conforms to the true value or concentration of the analyate

▪ Result of 19 → 21 is no difference

▪ Because it does not exceed the error of measurement of the assay and is therefore not real

▪ Every measurement or assay has some associated error of measurement and should be taken into account when evaluating a number or a result

▪ (True Positive + True Negative)/Total

▪

Precision of a lab test:

▪ definition - the quality of being sharply defined or stated, one measure of precision is the number of distinguishable alternatives to a measurement

▪ precision of a laboratory result refers to the reproducibility of a value

▪ it is quantified in terms of the coefficient of variation (CV), which is defined as the standard deviation divided by the mean of result values

Example of accuracy and precision:

The laboratory that you are using assays a serum sample that has a known cholesterol concentration of 200mg/dl. This analysis is performed 50 times and the following results are obtained:

Mean = 205mg/dl

SD = 10mg/dl

CV = 5%

Therefore the accuracy is +/- 10mg and the precision is 5%

Sensitivity of a lab test:

▪ sensitivity is the frequency of abnormal or positive test results in individuals who have a selected disease (% of true positive tests)

▪ True Positive/(True Positive + False Negative)

Specificity of a lab test:

▪ it is the frequency with which a normal or negative test result is observed in individuals free of the disease (% of true negative tests)

▪ sensitivity and specificity usually have an inverse relationship

▪ True Negative/(True Negative + False Positive)

Example of sensitivity and specificity:

A new pregnancy test was developed that was claimed by the manufacture to be better than existing tests. A clinical trial of this test was performed to evaluate this claim. For this study, 1000 females were tested. Of these subjects 900 were not pregnant and 100 were pregnant.

| |pregnant |non-pregnant |

|# of positives |90 |180 |

|# of negatives |10 |720 |

This pregnancy test is 90% sensitive 90/(90+10) , 80% specific 720/(720+180).

Intra-individual variation:

▪ One factor not controllable by either the physician or laboratory is the degree of normal diurnal, day-to-day, and seasonal variation in the concentration of measured components in biological fluids

▪ While the intra-individual variation of analytes such as electrolytes and serum proteins is generally considered to be small, the serum or urinary content of other biological components such as enzymes, lipids, hormones, and iron may change considerably over both shorter and longer time intervals

▪ It is important to consider the daily habits of a patient, such that the proper interpretation of results, as well as a guide in the decision of when to obtain a specimen for testing, can result

▪ Determination of changes in patients results must take into account two factors that have nothing to do with real clinical changes in the patient (analytical variation and intra-individual day-to-day variation)

Examples of intra-individual variation:

- Insulin levels are higher in the morning than later in the day.

- TSH levels are highest around or after midnight and lowest at midday. TSH is also

secreted in multiple pulses that occur throughout the 24 hr period.

Normal Values:

▪ “reference values”

▪ A subjective term that implies different meanings

o Most suited to survival, optimal, fittest

o Carrying no penalty, harmless

o Most representative of its class, average

▪ The average value & standard deviation are determined and a +/- 2 standard deviation range is calculated. This range accounts for 95% of the test results within the population and defines the limits of normal

▪ Ranges are dependent on population demographics, environment, analytical method

Decision Levels:

▪ In order to make a decision correctly, for a given test, decision levels for that test must be established

▪ Decision levels are values of a measured quantity which enable you to distinguish and individual with a disease from a reference population of similar individuals without the disease

▪ For the most part, these values are determined be each clinician, based on their clinical experience, practice patterns, and habit

▪ There are no tables or charts of decision levels, however there are exceptions, or generally accepted guidelines for a few measures

▪ When a lab result is complete and the information is presented to the patient, it is typically the patient that makes the final decision

Example of decision level:

Knowing the normal range of glucose (80-115)mg/dl, if a patient presents with a glucose level greater than 120, one can suspect that the patient may have diabetes.

Critical or panic values:

▪ These levels are so abnormal that they represent a threat to life.

▪ These lab results must be presented to the physician in an orderly yet timely manner (the lab will typically contact the physician.

Examples of panic values:

Ca below 6 or above 12

WBC below 3000

pH too high or too low

True Positive lab test:

▪ Percentage of true positive tests results in patients who have a disease you are looking for

True Negative lab test:

▪ Percentage of true negative tests results in patients who are free of the disease you are looking for

False Negative lab test:

▪ Ex: testing glucose – a false negative can result if urine is left to sit at room temperature for an extended period, owing to the rapid glycolysis of glucose

False Positive lab test:

▪ Interfering factors that can result positive (drugs, other disease, error)

Positive Predictive Value:

▪ True Positive/(True Positive + False Positive)

Negative Predictive Value;

▪ True Negative/(True Negative + False Negative)

List/give example of diseases/conditions that are associated with abnormal levels of:

Amylase:

▪ Enzyme that changes starch to sugar, produced in salivary glands and pancreas

▪ Typically tests for acute pancreatitis

▪ Also seen in:

o Cerebral trauma

o Inflammation of salivary duct

o Ruptured tubal pregnancy

o Pancreatic carcinoma

Leukocytosis: (table 2.2)

▪ Toxic granulation

▪ LE (lupus)

▪ Tart cell

▪ Chediak-Higashi anomaly

Rheumatoid Factor:

▪ Are known as anti-gamma-globulin antibodies

▪ Conditions:

o Morning stiffness at least 6 wks

o Pain on motion in joints

o Swelling in joint

o Subcutaneous nodules

▪ RF also present in:

o SLE

o TB

o Syphilis

o Cancer

o Viral infections

Uric Acid:

▪ Formed from breakdown of nucleonic acids and is end product of purine metabolism

▪ Overproduction occurs when there is excessive cell breakdown and catabolism of nucleonic acids (gout), excessive production and destruction of cells (leukemia) or an inability to excrete the substance produced (renal failure)

▪ Overproduction found in:

o Gout

o Renal diseases

o Alcoholism

o Down syndrome

o Lead poisoning

o Starvation

Glucose in the urine:

▪ Occurs in:

o Diabetes mellitus

o Endocrine disorders

o Liver and pancreatic disease

o CNS disorders

o Impaired tubular reabsorption

o Gestational diabetes

HCG:

▪ Human Chorionic Gonadotropin

▪ Produced by young placental trophoblasts

▪ Typically tested in pregnancy tests

▪ Positive results occur in:

o Choriocarcinoma

o Hydatidiform mole

o Testicular and trophoblastic tumors in males

o Chorioepithelioma

o About 65% of ectopic pregnancies

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