A 72-year-old man with IgG kappa myeloma (paraprotein 58 g/L) has a routine profile on the chemistry analyzer. Results: phosphate 4.10 mmol/L (12.7 mg/dL; reference 0.80-1.50), calcium 2.36 mmol/L (9.5 mg/dL), creatinine 92 µmol/L (1.04 mg/dL) and potassium 4.2 mmol/L; the hemolysis, icterus and lipemia indices are all below their limits. The analyzer flags the phosphate reaction curve as abnormal, with a sudden absorbance rise as soon as reagent is added. He has no symptoms. After the serum is deproteinized, phosphate is 1.08 mmol/L. The most likely cause of the first result is:

Paraprotein precipitating in the reagent

Carryover from the previous sample

Phosphate retention from kidney failure

Phosphate released by in vitro hemolysis

A glucose method is recalibrated in the afternoon. Both QC levels then read about 10% below their means, and patient results are lower than expected. The calibrator vial is found to have stood uncapped on the bench since the morning. What is the most likely cause?

The calibrator concentrated by evaporation

The reagent has lost activity since calibration

Both control materials have deteriorated

The calibrator diluted by condensation

A photometric method fails calibration because every calibrator, including the highest, gives a much smaller absorbance change than expected. The most likely cause is:

Deteriorated or wrongly prepared reagent

Slight carryover between calibrator samples

A calibrator concentrated by evaporation

Wrong calibrator values typed into the analyzer

An outpatient sample has a hemolysis index above the manufacturer's limit for potassium; the other requested tests are unaffected at this index. What should the laboratory do with the potassium?

Withhold it and request a new sample

Report it after a hemolysis correction formula

Dilute the sample and report that result

Report it unchanged with no comment

Chemistry automation and troubleshooting

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on Chemistry automation and troubleshooting, part of Clinical Chemistry. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 72-year-old man with IgG kappa myeloma (paraprotein 58 g/L) has a routine profile on the chemistry analyzer. Results: phosphate 4.10 mmol/L (12.7 mg/dL; reference 0.80-1.50), calcium 2.36 mmol/L (9.5 mg/dL), creatinine 92 µmol/L (1.04 mg/dL) and potassium 4.2 mmol/L; the hemolysis, icterus and lipemia indices are all below their limits. The analyzer flags the phosphate reaction curve as abnormal, with a sudden absorbance rise as soon as reagent is added. He has no symptoms. After the serum is deproteinized, phosphate is 1.08 mmol/L. The most likely cause of the first result is:
1. ACarryover from the previous sample
2. BPhosphate retention from kidney failure
3. CParaprotein precipitating in the reagent
4. DPhosphate released by in vitro hemolysis
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Answer: C · Paraprotein precipitating in the reagent
At high concentration, some paraproteins precipitate in the acidic phosphomolybdate reagent, and the turbidity is read as extra absorbance. The sudden absorbance jump at reagent addition, the abnormal reaction-curve flag and a normal result after deproteinization all point to this pseudohyperphosphatemia. Normal indices exclude hemolysis and lipemia, and a normal creatinine excludes renal retention. Carryover adds only a small fraction of the previous sample and would not cause an abnormal reaction curve. The phosphate should be reported from the deproteinized sample or by another method, with a comment, and other photometric tests such as bilirubin and HDL cholesterol can be affected in the same way.
- A: Carryover adds only a small fraction of the previous sample and does not explain the abnormal reaction curve.
- B: Creatinine is normal, and the phosphate is normal once protein is removed.
- D: The hemolysis index is below its limit, and the deproteinized result is normal.
Question 2 · Hard
A glucose method is recalibrated in the afternoon. Both QC levels then read about 10% below their means, and patient results are lower than expected. The calibrator vial is found to have stood uncapped on the bench since the morning. What is the most likely cause?
1. AThe reagent has lost activity since calibration
2. BBoth control materials have deteriorated
3. CThe calibrator diluted by condensation
4. DThe calibrator concentrated by evaporation
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Answer: D · The calibrator concentrated by evaporation
When water evaporates from an open calibrator, its true concentration rises above its assigned value. The analyzer links the larger signal to the smaller assigned value, so the calibration factor is too low and every sample, including the controls, reads low by about the same percentage. Use fresh calibrator, recalibrate and repeat QC before releasing results, and review patient results reported since the faulty calibration. Calibrators and controls should be kept capped and used within their open stability.
- A: A calibration done just now already allows for the reagent's current activity.
- B: Deteriorated controls would not make patient results low as well.
- C: A diluted calibrator would make results read high, not low.
Question 3 · Medium
A photometric method fails calibration because every calibrator, including the highest, gives a much smaller absorbance change than expected. The most likely cause is:
1. ASlight carryover between calibrator samples
2. BA calibrator concentrated by evaporation
3. CWrong calibrator values typed into the analyzer
4. DDeteriorated or wrongly prepared reagent
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Answer: D · Deteriorated or wrongly prepared reagent
The absorbance change from each calibrator depends on the reagent working properly. If the reagent has expired, been stored warm, stayed on board past its stability or been reconstituted wrongly, every calibrator gives less signal and the analyzer rejects the calibration. Check the reagent's lot, expiry, on-board time and preparation, replace it, recalibrate, and pass QC before testing patients. A concentrated calibrator would give more signal, not less.
- A: Carryover adds a little signal to one sample; it cannot cut the signal of every calibrator.
- B: Evaporation concentrates a calibrator and would increase its signal.
- C: Wrong assigned values change the calculated factor, not the measured absorbance.
Question 4 · Medium
An outpatient sample has a hemolysis index above the manufacturer's limit for potassium; the other requested tests are unaffected at this index. What should the laboratory do with the potassium?
1. AReport it after a hemolysis correction formula
2. BDilute the sample and report that result
3. CReport it unchanged with no comment
4. DWithhold it and request a new sample
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Answer: D · Withhold it and request a new sample
Red cells contain about 25 times more potassium than plasma, so even modest hemolysis in the tube raises the result. Once the index passes the limit, the error is larger than the method allows, so the result is withheld with an explanatory comment and a new sample is requested. Correction formulas are not recommended, because the potassium released varies between patients and the index cannot tell hemolysis in the tube from hemolysis in the body. Tests unaffected at this index can still be reported.
- A: Correction formulas are unreliable because the potassium released per unit of hemoglobin varies.
- B: Dilution lowers hemoglobin and potassium alike and does not remove the error.
- C: A falsely high potassium released without comment could lead to harmful treatment.
Question 5 · Hard
A sample's hemolysis index equals 1.0 g/L (100 mg/dL) free hemoglobin. The laboratory's limits, above which a result is withheld, are: potassium 0.5 g/L, LD 0.3 g/L, AST 0.8 g/L, sodium 10 g/L, creatinine 5 g/L. Which results can be reported?
1. ASodium and creatinine only
2. BSodium, creatinine and AST
3. CAll except potassium
4. DNone of the five results
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Answer: A · Sodium and creatinine only
Each test has its own hemolysis limit, set from interference studies, so the index is compared with each limit separately. At 1.0 g/L free hemoglobin, potassium, LD and AST all exceed their limits, because red cells contain much more of each than plasma; these are withheld with a comment and a new sample is requested. Sodium and creatinine are below their limits and can be reported. Applying one limit to every test would either lose valid results or release wrong ones.
- B: AST's limit of 0.8 g/L is exceeded at 1.0 g/L, so AST must be withheld.
- C: LD and AST also exceed their limits, not only potassium.
- D: Sodium and creatinine are below their limits and can be reported.
Question 6 · Easy
Automated chemistry analyzers estimate the hemolysis, icterus and lipemia indices of a sample by:
1. AMeasuring its electrical conductivity
2. BReading its absorbance at several wavelengths
3. CComparing its color with a printed chart
4. DRunning separate hemoglobin and bilirubin assays
Answer and explanation in LearnFree account
Question 7 · Hard
An ALT carryover study runs a high sample three times, then a low sample three times. Results (U/L): high 3,980, 4,010, 4,000; low 52, 44, 44. With carryover % = (low1 - low3) ÷ (high3 - low3) x 100, after a sample with ALT 12,000 U/L the next sample will be falsely raised by about:
1. A8 U/L
2. B2.4 U/L
3. C240 U/L
4. D24 U/L
Answer and explanation in LearnFree account
Question 8 · Easy
Which analyzer feature mainly prevents sample-to-sample carryover?
1. AKeeping reagents refrigerated on board
2. BRunning two control levels each shift
3. CReading each reaction at two wavelengths
4. DWashing the sample probe inside and out
Answer and explanation in LearnFree account
Question 9 · Medium
A sample has a high icterus index. On the same analyzer, which result is most likely to be falsely low?
1. AChloride by indirect ion-selective electrode
2. BGlucose by the hexokinase method
3. CCreatinine by an enzymatic peroxidase method
4. DSodium by indirect ion-selective electrode
Answer and explanation in LearnFree account

Chemistry automation and troubleshooting

A glucose method is recalibrated in the afternoon. Both QC levels then read about 10% below their means, and patient results are lower than expected. The calibrator vial is found to have stood uncapped on the bench since the morning. What is the most likely cause?

A photometric method fails calibration because every calibrator, including the highest, gives a much smaller absorbance change than expected. The most likely cause is:

An outpatient sample has a hemolysis index above the manufacturer's limit for potassium; the other requested tests are unaffected at this index. What should the laboratory do with the potassium?

A sample's hemolysis index equals 1.0 g/L (100 mg/dL) free hemoglobin. The laboratory's limits, above which a result is withheld, are: potassium 0.5 g/L, LD 0.3 g/L, AST 0.8 g/L, sodium 10 g/L, creatinine 5 g/L. Which results can be reported?

Automated chemistry analyzers estimate the hemolysis, icterus and lipemia indices of a sample by:

An ALT carryover study runs a high sample three times, then a low sample three times. Results (U/L): high 3,980, 4,010, 4,000; low 52, 44, 44. With carryover % = (low1 - low3) ÷ (high3 - low3) x 100, after a sample with ALT 12,000 U/L the next sample will be falsely raised by about:

Which analyzer feature mainly prevents sample-to-sample carryover?

A sample has a high icterus index. On the same analyzer, which result is most likely to be falsely low?