Path- Judicious Use of the Clinical Laboratory

Question 1

What are the 4 uses of laboratory testing?

Answer 1

1. screening
   - neonate inherited metabolic disorders
   - TSH
   - PSA in men
2. diagnosis
   - diabetes
   - troponin levels to diagnose MI
3. monitoring and following response to treatment
   - lipid levels after getting put on a statin
   - TSH to monitor hormone replacement therapy
4. prognosis
   - CEA [cancer antigens]
   - serum lipids for ischemic heart disease risk assessment
   - acetaminophen levels at 4 hrs for hepatic toxicity

Question 2

What characteristic must a test have for it to be used as a screening test?

Answer 2

It must be of high predictive value that is obviously affected by prevalence.

Question 3

What can improve the predictive value of a screening test?

Answer 3

Predictive value can be improved by increasing the prevalence by selecting a target population likely to have the disorder on which the test is performed.

Question 4

What levels are screened during the first and second trimester to screen for congenital anomalies like Down syndrome and neural tubular defects?

What confirmatory tests are done for positive screens?

Answer 4

1. hCG
2. a-fetoprotein
3. dimeric Inhibin A
4. unconjugated estratriol

If you get a positive screen, confirm with:

1. biochemical tests
2. genetic tests
3. amniocentesis or chorionic villi sampling

Question 5

How many disorders does the Texas neonate screening test for?

Answer 5

29

Question 6

What was the first inborn error of metabolism screened for through population-based screening?

Answer 6

phenylalanine hydroxylase deficiency leading to phenylketonuria

Question 7

Why is it imperative to screen for phenylalanine hydroxylase deficiency?

What will levels be in the classic form where there is complete enzyme deficiency?
What level is necessary to be diagnostic?

Answer 7

Neonates with this deficiency have no physical signs except low birth weight and smaller head circumference.

But if they keep accumulating phenylalanine in the diet, it can lead to brain damage, and impaired cognitive development.

Complete deficiency will have levels over 1200micmol/L, but there is a rance. Anything over 130 is diagnostic.

Question 8

Why is there a lot of false positives in newborn screening of PKU?
What can be done to reduce the number of FP?

Answer 8

The tests are extremely sensitive [TP/TP+FN], but in order to have this sensitivity, there will be a non-specific elevation in false positives.

FP can be reduced by using phenylalanine/tyrosine ratio as a surrogate marker. Ratio over 3 is abnormal

Question 9

Not including newborn screening, what are the major screening tests for the general population?

Answer 9

1. TSH [because there is high prevalence in women, but the presentation can be subtle]
2. PSA
3. drugs of abuse pre-employment
4. glucose/lipids during annual health exam for diabetes and dyslipidemias

Question 10

How can lab tests diagnose diabetes?

Answer 10

Fasting glucose over 100 or post-prandial over 200 is diagnostic for diabetes

Question 11

How can lab tests diagnose hypercholesterolemia?

Answer 11

levels greater than 200mg/dl

Question 12

How can the lab diagnosis of Hashimoto’s thyroiditis be made?

Answer 12

1. TSH over 4.5mIU/L

2. anti-thyroid Ab

Question 13

When using lab tests for monitoring, what should values of the following be:

1. HbA1c
2. phenylalanine
3. TSH
4. intraoperative PTH
5. PSA

Answer 13

1. below 6.5
2. below 300micmol/L
3. 0.5-2.5mU/L
4. decrease by 50% in the first 5-10 minutes after resection of parathyroid gland
5. undetectable

Question 14

What are examples of when lab tests are used for prognosis?

Answer 14

1. serial measurements of plasma Cr and BUN for progression of renal failure which indicates when to start dialysis
2. total cholesterol, HDL, age, smoking, gender to give prognosis for future coronary artery disease
3. urine albumin for progression of diabetic nephropathy
4. CEA and other tumor markers can help assess prognosis and staging of disease

Question 15

What is the equation for sensitivity and specificity?

Answer 15

Sensitivity = [TP/TP+FN] x 100

Specificity = [TN/TN+FP] x 100

Question 16

What is efficiency of a lab test?

Answer 16

percent of people classified correctly and having OR not having the disease.

TN+TP / [TOTAL] x 100

Question 17

What are the 3 major components of the laboratory testing cycle?

Answer 17

1. pre-analytial
   - decide to perform test
   - test order placed
   - patient info entered
   - patient prep and sample collection
2. analytical
   - specimen processed
   - analyte measured
3. post-analytical
   - report generated
   - results interpreted
   - data interpreted
   - clinical response

Question 18

Which stage of the laboratory testing cycle is responsible for the most errors?

Answer 18

Pre-analytical

• tube filling
• patient ID
• wrong collection tube
• test request
• empty tube
• other

Question 19

What are the controllable variables of the pre-analytic phase?
What are the uncontrollable variables?

Answer 19

Controllable:

1. test selection
2. sample collection protocol
3. patient posture, diet, infusion,transfusion, exercise

Not controllable:

1. circadian rhythm
2. biological variability
3. age, gender
4. disease state

Question 20

What tests are affected by patient posture?

Answer 20

serum proteins, aldosterone, renin due to change in volume distribution

Question 21

What lab test is affected pre-analytically by:

1. dietary factors of the patient
2. alcohol intake
3. physical exercise
4. stress

Answer 21

1. lipids
2. GGT
3. CK
4. cortisol and prolactin

Question 22

Digoxin, theophylline, phenytoin and cyclosporine are best measured when?

Answer 22

They are best measured at trough level [lowest level of a drug] so it is imperative to know the time of last dose

Question 23

Time of collection is important for various enzymes, metabolites, vitamins and drugs.
What are three examples of cyclic time scales?

Answer 23

1. circadian - cortisol has diurnal rhythm with highest level in morning and nadir in evening
2. biological/pregnancy
   - LH and FSH change markedly during the follicular and luteal phases of the menstrual cycle
   - cortisol due to binding globulin changes in pregnancy
3. seasonal changes- vitamin D is higher in the summer due to increased sun exposure

Question 24

What order should tubes be collected when taking blood?

Answer 24

1. normal tube
2. heparin
3. fluoride oxalate
4. EDTA [ must be last because even 1 drop in any other tube will contaminate]

Question 25

What do heparin, EDTA, and fluoride oxalate tubes allow?

Answer 25

They prevent blood clotting which allows “whole blood specimen”.
This is best for analyzing:
1. hematological counts
2. levels of immunosuppressive drug cyclosporin

Question 26

After using whole blood for hematological counts, it is spun down and the supernatant is used for what?

Answer 26

Supernatant is plasma and it is used for renin levels

Question 27

If you know you need a glucose level, but getting it to the lab is going to take over 2 hours, what tube should you use?

Answer 27

Fluoride oxalate because it inhibits glycolysis

Question 28

If you want to measure the tumor marker CEA, what tube must you use?

Answer 28

Sodium heparin

Question 29

How is “serum” obtained?

How is “plasma” obtained?

Answer 29

Serum is obtained following clotting and centrifugation in tubes without additives.

Plasma is the liquid phase following centrifugation WITH additives

Question 30

__________ is not suitable for protein electrophoresis due to interference with fibrinogen.
_________ are not suitable for metabolic panels beause they chelate Ca and Mg, along with increasing K and Na erroneously.

Answer 30

Plasma is not suitable for electrophoresis

EDTA cannot be used for metabolic panels

Question 31

What are the laboratory terms for:

1. detection limit
2. interference
3. reproducibility
4. measuring range of the assay

Answer 31

1. sensitivity- lowest concentration the assay can reliably detect
2. specificity- degree of interference from other metabolites, anayltes, endogenous material [drugs/inhibitors]
3. precision - dependent on the assay and the concentration of the analyte
4. linearity- must cover the clinically useful range

Question 32

What are things that can decrease the specificity of a lab test?

Answer 32

Spectral

1. hemolysis
2. lipemia
3. bilirubin

Drug metabolites

Antibodies

1. HAMA
2. Heterophilic Ab

Question 33

The degree of precision of a lab test depends on what 2 factors?

Answer 33

1. assay

2. analyte concentration

Question 34

What are the 2 types of analytical lab errors?

Answer 34

1. random- caused by timing, temp, pipetting variations. INDEPENDENT of the operator performing the experiment
2. systematic- time-dependent change in instrument calibration that causes the calibration curve to shift its position, thus altering precision
   The two types are :
   - constant
   - proportional

Question 35

If you are running an assay for hCG with mouse monoclonal Ab, what will the presence of HAMA do?

Answer 35

Interfere with the assay and give a false positive on the pregnancy test

Question 36

In a normal individual what is the ratio of BUN/Cr?
What does an increased ratio usually indicate?

What substance can cause negative interference Cr?
What substance can cause positive interference to Cr?

Answer 36

Normal is 10:1 to 20:1
If the ratio is greater than 20:1 it indicates pre-renal cause of uremia.

Bilirubin falsely lowers Cr measurement giving the appearance of a higher BUN/Cr ratio

Ketones falsely elevate Cr giving the appearance of a lower BUN/Cr

Question 37

What are examples of post-analytic errors?

Answer 37

1. transcription errors
   - wrong patient
   - wrong value and/or wrong unit
2. using wrong reference ranges [because they vary by gender, age, season, gestation, etc]