Instrumentation

Question 1

What colligative property is most commonly used in the clinical lab to determine osmolality?

Answer 1

Freezing point depression.

Question 2

What does the colligative property “freezing point depression” depend on?

Answer 2

The number of solute particles in a solution, not their identity or weight.

Question 3

Describe the first step in the freezing point depression method for measuring osmolality.

Answer 3

The sample is supercooled to a temperature below its freezing point.

Question 4

How is freezing initiated in the supercooled sample during the freezing point depression method?

Answer 4

By a mechanical vibration or agitation.

Question 5

How is the osmolality value ultimately determined in the freezing point depression method?

Answer 5

The instrument measures the temperature change (the depression) from the freezing point of pure water (0°C) and calculates osmolality based on that value.

Question 6

The amount of freezing point depression is proportional to what?

Answer 6

The number of osmotically active particles present in the sample.

Question 7

Why is freezing point depression preferred over boiling point elevation for clinical osmolality measurements?

Answer 7

Because heating a clinical sample (like serum or urine) could cause decomposition of its components, leading to an inaccurate result.

Question 8

In the qualitative uhCG TestPack for pregnancy, what does the positive control line indicate?

Answer 8

The positive control line verifies that the test device and procedure are working correctly. It should appear on every valid test, whether the patient is pregnant or not. It is not an indicator of pregnancy itself.

Question 9

Why is the uhCG TestPack specific for the hCG hormone and less likely to cross-react with other hormones like LH or FSH?

Answer 9

It uses an antibody specific to the β-subunit of hCG (anti-β-hCG). This unique subunit is not shared by other glycoprotein hormones, providing high specificity.

Question 10

What is a key limitation of the qualitative hCG pregnancy test?

Answer 10

It cannot distinguish between hCG from a pregnancy and hCG produced by certain trophoblastic tumors (e.g., choriocarcinoma). A positive result may be due to a tumor, not a viable pregnancy.

Question 11

How does the qualitative hCG pregnancy test indicate that an interferent (like excess urine protein) may have made the result unreadable or invalid?

Answer 11

An “X” appears in the negative control area of the result window. This signals an invalid test that must be repeated with a new device.

Question 12

What is the Analytical Measuring Range (AMR)?

Answer 12

The range of values an instrument can report directly without any dilution or concentration of the sample.

Question 13

What is the Clinically Reportable Range (CRR)?

Answer 13

The range of values an instrument can report as a quantitative result, which may require sample dilution or concentration. It is wider than the AMR. Values outside this range are reported as “greater than” (>) or “less than” (<).

Question 14

What is the Reportable Range?

Answer 14

The overall range of values that a lab can confidently report as valid results. It is based on the defined limits set by the lab, which include both the AMR and the CRR.

Question 15

On a Vitros analyzer, what does a flag for an AST result of “>750 U/L” typically indicate?

Answer 15

It indicates that the analyte’s concentration has exceeded the upper limit of the instrument’s Analytical Measurement Range (AMR). The result is higher than the instrument can accurately quantify.

Question 16

What is the primary next step when a patient sample produces a result flagged for exceeding the upper AMR?

Answer 16

The sample must be diluted with an appropriate diluent and then re-analyzed. The final result is calculated by multiplying the diluted result by the dilution factor.

Question 17

A ferritin sample gives a result of >1500 µg/L. A 1:2 (x2) dilution is performed, and the analyzer reads 761 µg/L. What is the final result you should report?

Answer 17

1522 µg/L

Question 18

What does ISE stand for in clinical chemistry?

Answer 18

Ion Selective Electrode.

Question 19

How does an Ion Selective Electrode (ISE) work?

Answer 19

It allows for the potentiometric measurement of a specific ion using a selective membrane that is permeable only to the target ion based on its size and charge, preventing interference from other ions.

Question 20

What is the key characteristic of Direct ISE?

Answer 20

The ion activity is measured directly in the undiluted sample (e.g., whole blood or serum).

Question 21

What is the key characteristic of Indirect ISE?

Answer 21

The sample is diluted with a large volume of buffer solution before the measurement is taken.

Question 22

What is the main practical difference between Direct and Indirect ISE?

Answer 22

Direct ISE measures in an undiluted sample, while Indirect ISE requires the sample to be diluted first. This can lead to different results, especially in patients with abnormal protein or lipid levels.

Question 23

Why can Direct and Indirect ISE methods give different results for electrolyte levels in patients with abnormal protein or lipid levels?

Answer 23

This discrepancy is due to the volume displacement effect (also called the electrolyte exclusion effect).

Indirect ISE dilutes the sample, which corrects for this effect and reports concentration in the plasma water fraction. In lipemic or hyperproteinemic samples, the non-aqueous portion (fats, proteins) of the blood is larger, so the concentration of electrolytes in the total plasma volume is artificially low.

Direct ISE measures the activity in the undiluted sample and is largely unaffected by the volume of lipids and proteins. It more accurately reflects the true physiologically active ion concentration in the aqueous phase of plasma.