A small laboratory runs a manual colorimetric phosphorus method for a kidney clinic's batch of samples. The photometer is zeroed on a reagent blank, and five standards give these absorbances: 2.0 mg/dL [0.65 mmol/L], 0.101; 4.0 mg/dL [1.29 mmol/L], 0.204; 6.0 mg/dL [1.94 mmol/L], 0.243; 8.0 mg/dL [2.58 mmol/L], 0.405; 10.0 mg/dL [3.23 mmol/L], 0.508. The package insert claims linearity to 15 mg/dL [4.84 mmol/L], and the controls have not yet been run. The first patient's sample reads 0.300. The technologist should:

Remake the 6.0 mg/dL standard and recalibrate

Report the result read from a line through all points

Dilute the sample, as the curve bends above 4 mg/dL

Open a new reagent lot, as the slope is too shallow

Glucose standards of 0, 50, 100, 200, 300 and 400 mg/dL [0 to 22.2 mmol/L] read 0.000, 0.110, 0.220, 0.440, 0.600 and 0.700. A patient's sample reads 0.640. The best action is to:

Dilute the sample and assay it again

Report 340 mg/dL [18.9 mmol/L], read from the curve

Report it as above 400 mg/dL [22.2 mmol/L]

Report 291 mg/dL [16.2 mmol/L], using the slope

When a method follows Beer's law and the photometer is zeroed on a reagent blank, plotting absorbance against concentration on linear graph paper gives:

A straight line through the origin

A curve that falls exponentially

A straight line that slopes downward

A curve that rises, then levels off

Standard curves

7 ASCP MLS practice questions on Standard curves, part of Laboratory Operations. The first 7 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 small laboratory runs a manual colorimetric phosphorus method for a kidney clinic's batch of samples. The photometer is zeroed on a reagent blank, and five standards give these absorbances: 2.0 mg/dL [0.65 mmol/L], 0.101; 4.0 mg/dL [1.29 mmol/L], 0.204; 6.0 mg/dL [1.94 mmol/L], 0.243; 8.0 mg/dL [2.58 mmol/L], 0.405; 10.0 mg/dL [3.23 mmol/L], 0.508. The package insert claims linearity to 15 mg/dL [4.84 mmol/L], and the controls have not yet been run. The first patient's sample reads 0.300. The technologist should:
1. AReport the result read from a line through all points
2. BDilute the sample, as the curve bends above 4 mg/dL
3. COpen a new reagent lot, as the slope is too shallow
4. DRemake the 6.0 mg/dL standard and recalibrate
Show answerHide answer
Answer: D · Remake the 6.0 mg/dL standard and recalibrate
Four of the five standards give about 0.051 absorbance units per mg/dL, a straight line through zero, so the method is linear across the range. The 6.0 mg/dL standard reads 0.243 instead of about 0.305, roughly 20% low, which points to an error in preparing or pipetting that standard rather than a problem with the method. A calibration that contains a clearly wrong point must not be used, and the patient's result sits right where the error matters; read from the four good points, 0.300 would be about 5.9 mg/dL [1.9 mmol/L]. The standard should be remade, the method recalibrated and the controls run before any patient result is reported.
- A: A calibration with a clearly wrong point must not be used; the faulty standard has to be corrected first.
- B: The 8.0 and 10.0 mg/dL standards lie on the line, so the method is linear; only one point is off.
- C: Deteriorated reagent lowers every standard's absorbance, not just one in the middle.
Question 2 · Easy
The dashed line on the biuret calibration curve shown marks the absorbance of a patient's serum. The patient's total protein is:
Figure: MedExams
1. A8.0 g/dL [80 g/L]
2. B6.0 g/dL [60 g/L]
3. C3.5 g/dL [35 g/L]
4. D7.0 g/dL [70 g/L]
Show answerHide answer
Answer: D · 7.0 g/dL [70 g/L]
Reading across from the serum's absorbance of 0.35 to the calibration line and down gives 7.0 g/dL [70 g/L]. The line passes through the origin with a slope of 0.05 absorbance per g/dL, as Beer's law predicts, so 0.35 / 0.05 = 7.0 g/dL. Results are valid only between the lowest and highest calibrators; a sample above the top standard is diluted and retested.
- A: 8.0 g/dL reads 0.40 on this line.
- B: 6.0 g/dL reads 0.30 on this line.
- C: 3.5 is the absorbance times ten, read from the wrong axis.
Question 3 · Hard
The dashed lines on the glucose calibration shown mark a patient's serum read undiluted and again after diluting 1 part serum with 4 parts saline. What glucose should be reported?
Figure: MedExams
1. A700 mg/dL [38.9 mmol/L]
2. B950 mg/dL [52.7 mmol/L]
3. C760 mg/dL [42.2 mmol/L]
4. D190 mg/dL [10.5 mmol/L]
Show answerHide answer
Answer: B · 950 mg/dL [52.7 mmol/L]
The calibration is linear to at least 400 mg/dL and then flattens, so the undiluted reading of 1.15 lies on the flat part and its apparent 700 mg/dL is falsely low. The diluted reading of 0.38 lies on the linear part, at 0.002 absorbance per mg/dL: 0.38 / 0.002 = 190 mg/dL. One part serum plus 4 parts saline is a 1:5 dilution, so the result is 190 x 5 = 950 mg/dL [52.7 mmol/L]. Multiplying by 4 counts only the diluent, and 190 mg/dL leaves out the dilution factor.
- A: 700 mg/dL is read from the flat part of the curve, beyond the linear range.
- C: 760 mg/dL multiplies by 4, counting only the parts of saline.
- D: 190 mg/dL is the diluted result before the dilution factor is applied.
Question 4 · Hard
Glucose standards of 0, 50, 100, 200, 300 and 400 mg/dL [0 to 22.2 mmol/L] read 0.000, 0.110, 0.220, 0.440, 0.600 and 0.700. A patient's sample reads 0.640. The best action is to:
1. ADilute the sample and assay it again
2. BReport 340 mg/dL [18.9 mmol/L], read from the curve
3. CReport it as above 400 mg/dL [22.2 mmol/L]
4. DReport 291 mg/dL [16.2 mmol/L], using the slope
Show answerHide answer
Answer: A · Dilute the sample and assay it again
Up to 200 mg/dL the absorbance rises by 0.110 for every 50 mg/dL, but above that each 100 mg/dL step adds only 0.160 and then 0.100 instead of 0.220. The method is therefore linear only to about 200 mg/dL, and 0.640 lies in the flattened part, where small absorbance errors cause large concentration errors. Diluting the sample brings it into the linear range, and the result is then multiplied by the dilution factor. A value read beyond the linear range, by slope or by curve, is not reportable.
- B: 340 mg/dL comes from the flattened part of the curve, where readings are unreliable.
- C: 0.640 is below the 400 mg/dL standard's 0.700, so the sample is not above 400 mg/dL.
- D: 291 mg/dL applies the linear slope where the response is no longer linear.
Question 5 · Easy
When a method follows Beer's law and the photometer is zeroed on a reagent blank, plotting absorbance against concentration on linear graph paper gives:
1. AA curve that falls exponentially
2. BA straight line through the origin
3. CA straight line that slopes downward
4. DA curve that rises, then levels off
Show answerHide answer
Answer: B · A straight line through the origin
Beer's law states that absorbance is directly proportional to concentration, so absorbance against concentration is a straight line. Zeroing on a reagent blank sets zero concentration to zero absorbance, so the line passes through the origin. Percent transmittance falls exponentially with concentration and gives a straight downward line only on semilog paper. A curve that flattens at the top shows that the method's linear limit has been passed.
- A: An exponential fall is how %T against concentration looks on linear paper.
- C: A straight downward line is %T against concentration on semilog paper.
- D: Flattening at high concentration is a departure from Beer's law, not its normal shape.
Question 6 · Hard
A glucose method is calibrated with two calibrators: 50 mg/dL [2.8 mmol/L] reads 0.180 and 250 mg/dL [13.9 mmol/L] reads 0.580, with a linear response between them. A diabetic patient's sample reads 0.420. What is the patient's glucose?
1. A170 mg/dL [9.4 mmol/L]
2. B210 mg/dL [11.7 mmol/L]
3. C181 mg/dL [10.0 mmol/L]
4. D117 mg/dL [6.5 mmol/L]
Answer and explanation in LearnFree account
Question 7 · Medium
Calibrators for a competitive immunoassay give an S-shaped curve of signal against log concentration. How should patient results be calculated from it?
1. AWith linear regression on all calibrators
2. BWith a four-parameter logistic curve fit
3. CWith a line from zero to the top calibrator
4. DAs a ratio to one mid-range calibrator
Answer and explanation in LearnFree account

Standard curves

The dashed line on the biuret calibration curve shown marks the absorbance of a patient's serum. The patient's total protein is:

The dashed lines on the glucose calibration shown mark a patient's serum read undiluted and again after diluting 1 part serum with 4 parts saline. What glucose should be reported?

Glucose standards of 0, 50, 100, 200, 300 and 400 mg/dL [0 to 22.2 mmol/L] read 0.000, 0.110, 0.220, 0.440, 0.600 and 0.700. A patient's sample reads 0.640. The best action is to:

When a method follows Beer's law and the photometer is zeroed on a reagent blank, plotting absorbance against concentration on linear graph paper gives:

A glucose method is calibrated with two calibrators: 50 mg/dL [2.8 mmol/L] reads 0.180 and 250 mg/dL [13.9 mmol/L] reads 0.580, with a linear response between them. A diabetic patient's sample reads 0.420. What is the patient's glucose?

Calibrators for a competitive immunoassay give an S-shaped curve of signal against log concentration. How should patient results be calculated from it?