Laboratory medicine and the pathology report
Introduit| English | Sens |
|---|---|
| reference interval | The central 95% of results from healthy people (2. |
| haemolysis/hiːˈmɒləsɪs/ | The rupture of red blood cells, releasing their contents into the surrounding fluid. |
| surgical margin | The distance from the tumour to the ink. |
A potassium that could kill, or a broken tube?
- A lab calls: potassium 8.2 mmol/L, a level that can stop the heart. The patient feels well and the ECG is normal. The sample sat for hours and looks pink.
- Before you ask what is wrong with the patient, ask what happened to the sample.
The reference interval
- A reference interval 参考区间 is the central 95% of results from healthy people (2.5th to 97.5th percentile), so 5% of healthy people fall outside it by definition.
- Order many tests and some healthy results will be flagged. Trends and the clinical picture decide what matters.
What percentage of healthy people fall outside a reference interval?
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The interval holds the central 95%, so 2.5% on each side fall outside.
Pre-analytical errors
- Haemolysis 溶血 bursts red cells and releases potassium: a falsely high K⁺. An EDTA tube used for chemistry makes K⁺ high and Ca²⁺ low.
- Glucose falls about 5–7% per hour in a tube that is not spun, because the cells keep using it. Most lab errors happen before the analyser.
A pink, haemolysed sample shows K⁺ 8.2 mmol/L in a well patient with a normal ECG. What should you do first?
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Burst red cells release potassium; check the sample before the patient.
Glucose in an unspun tube rises the longer it waits.
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It falls about 5–7% per hour because the blood cells keep using it.
Units
- Glucose in mmol/L × 18 = mg/dL. So 5.5 mmol/L is 99 mg/dL, the same concentration written two ways.
- A result read in the wrong unit is a phone call, not a diagnosis.
A fasting glucose is 7.0 mmol/L. What is it in mg/dL?
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7.0 × 18 = 126 mg/dL, the fasting threshold for diabetes.
The structured report
- A pathology report runs in stations: specimen, gross description, microscopy, diagnosis (with grade and stage), then a comment.
- Each station catches a different error: the wrong specimen, a missed lesion, an unclear diagnosis.
Put the sections of a pathology report in order.
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From what was received to what it means.
Margins
- The surgeon's specimen is inked on the outside. A surgical margin 手术切缘 is the distance from the tumour to the ink.
- Tumour touching the ink is a positive margin: some may be left behind, so more surgery or radiotherapy may follow.
Tumour cells touch the inked surface of the specimen. What is the margin?
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Tumour at the ink means some may remain in the patient.
Worked example: many tests, false alarms
- (Fictional) Each test flags 5% of healthy people, so a healthy person passes one test with probability 0.95.
- With 20 independent tests, the chance all are normal is 0.95²⁰ ≈ 0.36. About 64% of healthy people get at least one flag.
A healthy person has 10 independent tests, each with a 95% chance of being normal. What is the chance that all 10 are normal? Give a percentage to one decimal place.
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0.95¹⁰ ≈ 0.599, so about 59.9%; about 40% get at least one flag.
Worked example: three impossible results
- (Fictional) K⁺ 8.2 mmol/L with a normal ECG and a pink sample: haemolysis until proven otherwise; repeat it.
- Glucose 2.8 mmol/L after a long delay: the cells ate it. Calcium 1.9 mmol/L in an EDTA tube: the tube is wrong; redraw.
Three common slips
- Reading "outside the interval" as disease. 5% of healthy people are outside by definition.
- Trusting the analyser over the story. It measures the tube faithfully, not the patient.
- Ignoring the specimen line in a report. It anchors everything below it.
Recap
- Interval = central 95% of healthy people; 5% of healthy results fall outside.
- Pre-analytical first: haemolysis (K⁺ up), EDTA (K⁺ up, Ca²⁺ down), unspun glucose falls 5–7% per hour.
- Glucose mmol/L × 18 = mg/dL; report: specimen → gross → microscopy → diagnosis → comment; tumour at the ink = positive margin.