MLT GlobeLaboratory Operations: Quality control & QA
54 Laboratory Operations MCQs on Quality control & QA with answers and explanations.
A single control result outside ±3 SD violates the Westgard 1-3s rule and most often signals:
1-3s is a rejection rule mainly sensitive to random error (e.g. pipetting, bubbles). Patient results must not be released until the cause is found and corrected.
A control has a mean of 200 mg/dL and an SD of 5 mg/dL. The coefficient of variation is:
CV = SD ÷ mean × 100 = 5 ÷ 200 × 100 = 2.5%. CV lets you compare the precision of methods at different concentrations.
Out of 100 people with a disease, a test is positive in 90. The test's sensitivity is:
Sensitivity = true positives ÷ (true positives + false negatives) = 90 ÷ 100 = 90%. Specificity would need results from people without the disease.
In the classic Westgard multirule procedure, the 1-2s rule is used as:
In the classic multirule scheme, 1-2s is a warning that prompts checking the other rules. Using it alone for rejection causes a high false-rejection rate (about 5% per control).
Repeated control results are tightly grouped but far from the established target value. The method is best described as:
Close agreement between replicates means good precision; distance from the true value means poor accuracy (bias). Precision and accuracy are independent characteristics.
A potassium control has a mean of 4.0 mmol/L and an SD of 0.1 mmol/L. The ±2 SD acceptable range is:
±2 SD = 4.0 ± (2 × 0.1) = 3.8 to 4.2 mmol/L, which includes about 95% of expected values. 3.7–4.3 is the ±3 SD range.
The main purpose of proficiency testing (external quality assessment) is to:
Proficiency testing sends samples of unknown value to many labs and grades each result against peers or target values, assessing accuracy externally. Daily precision is monitored by internal QC.
A reference interval is usually defined as the central 95% of results from healthy people. Therefore, among healthy individuals:
The central 95% excludes 2.5% at each end, so about 1 in 20 healthy people will have an 'abnormal' result. 32% applies to a ±1 SD range.
A glucose result exceeds the upper limit of the analytical measurement range. The appropriate action is to:
Results outside the AMR are unverified. The sample should be diluted using a validated dilution and the result multiplied by the dilution factor, or reported as greater than the limit per policy.
A new test is performed on 200 people known to be free of the disease. It is negative in 170 and positive in 30. What is its specificity?
Specificity = true negatives / all people without disease = 170/200 = 85%. The false-positive rate is 30/200 = 15%.
A specimen is labelled with the wrong patient's name at the bedside. This error occurs in which phase of testing?
Patient identification and specimen labelling happen before analysis, so it is a pre-analytical error, the phase where most laboratory errors occur. Analytical errors happen during measurement.
A correct result is transcribed into the wrong patient's record during manual entry. This is an error in which phase?
Reporting, transcription and communication of results are post-analytical steps. The measurement itself was correct, so it is not an analytical error.
A potassium result of 6.9 mmol/L is a critical value. The best practice when reporting it by telephone is to:
Critical values need prompt communication to a responsible licensed caregiver, with read-back of the result and documentation of who received it. Waiting for the clinician to see it in the system may delay urgent care.
Which international standard specifies requirements for quality and competence of medical laboratories?
ISO 15189 is the specific standard for medical laboratory quality and competence used for accreditation. ISO 14001 covers environmental management, and ISO 9001 is a general quality standard.
Precision of a laboratory method refers to:
Precision is reproducibility; closeness to the true value is accuracy.
In a normal (Gaussian) distribution of control results, about what percentage of values lie within ±2 SD of the mean?
About 68% of values lie within ±1 SD, 95% within ±2 SD and 99.7% within ±3 SD.
Specificity of a diagnostic test is the proportion of:
Specificity = true negatives ÷ (true negatives + false positives); the proportion of diseased people testing positive is sensitivity.
Most laboratory errors occur in which phase of testing?
Errors in ordering, patient identification, collection, labelling and transport account for most laboratory errors.
A written, step-by-step document describing exactly how a test is performed in the laboratory is called a:
SOPs ensure every staff member performs the test the same way and are part of document control.
Seven consecutive control values fall on the same side of the mean, all within ±2 SD. This is a:
A sudden run of values on one side of the mean is a shift, often from a new reagent lot or calibration change. A gradual drift in one direction is a trend (e.g. lamp ageing).