CT Scan, MRI or Ultrasound: Which Test and Why
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A blood report with a few values marked high or low causes a great deal of unnecessary alarm. Here is what the common parameters actually mean.
Reference ranges are not verdicts
A reference range represents the values found in 95% of a healthy population. By definition, one in twenty healthy people falls outside it on any given test. A marginally abnormal value in a person who feels entirely well is often of no significance at all.
Ranges also vary between laboratories depending on the method and equipment used, which is why comparing a value against a different laboratory's range can mislead. Always interpret against the range printed on that report.
Worth knowing
Never start, stop or change medication based on a report alone. The pattern of several values, your symptoms and your history together tell the story — a single number does not.
Complete blood count
- Haemoglobin — low means anaemia; the MCV tells you whether it is iron deficiency (low), B12 or folate deficiency (high), or chronic disease
- White cell count — high suggests bacterial infection or inflammation; low can occur in viral infection, typhoid, dengue and some medications
- Platelets — low in dengue, viral fevers and some medication reactions; the trend matters more than a single value
- ESR and CRP — non-specific markers of inflammation, useful for tracking response but never diagnostic on their own
Liver and kidney panels
In the liver panel, ALT and AST reflect liver cell injury and are commonly raised in fatty liver, alcohol use and viral hepatitis. Alkaline phosphatase and GGT point towards bile duct problems. Bilirubin rises in jaundice, and albumin reflects the liver's synthetic capacity over time.
In the kidney panel, creatinine is the main marker, but it varies with muscle mass — a 55 kg elderly woman and a 90 kg young man with the same creatinine have very different kidney function. This is why eGFR, which adjusts for age and sex, is the more useful number. Urea rises with dehydration and gastrointestinal bleeding as well as kidney disease.
Sugar, lipids and thyroid
- Fasting glucose: normal below 100 mg/dL, prediabetes 100–125, diabetes 126 or above
- HbA1c: reflects three-month average sugar; below 5.7% normal, 6.5% or above diabetes
- LDL cholesterol: the primary treatment target, with the goal depending on overall cardiac risk
- Triglycerides: aim below 150 mg/dL; strongly influenced by diet, alcohol and diabetes
- TSH: the screening thyroid test; a high TSH indicates an underactive thyroid, a low TSH an overactive one
Preparing correctly so the result means something
- Fast 8–12 hours for glucose and lipid profile; water is allowed
- Fasting is not needed for CBC, HbA1c, thyroid, liver and kidney tests
- Avoid heavy exercise the day before — it raises muscle enzymes and some other markers
- Avoid alcohol for 24 hours before liver and lipid testing
- Tell the laboratory about medicines and supplements; biotin in particular distorts thyroid and hormone assays
- Test at the same time of day when tracking a value over months, since several vary diurnally
- Carry previous reports — trend is far more informative than a single point
When an abnormal result genuinely matters
Values that are markedly rather than marginally abnormal, values that are changing progressively across serial reports, and abnormal values accompanied by symptoms all warrant proper evaluation.
Critical values — a very low platelet count, dangerously high potassium, a positive blood culture — are telephoned directly to the treating doctor by our laboratory rather than waiting for the printed report to be collected.
Key takeaways
- One in twenty healthy people falls outside any given reference range by definition.
- eGFR is more informative than creatinine alone, because creatinine varies with muscle mass.
- Fasting is required for glucose and lipids, but not for HbA1c, thyroid or blood counts.
- Trends across serial reports are far more meaningful than any single value.
A note on medical advice
This article is general health education written and reviewed by the Departments of Radiology & Pathology at SIMS. It is not a substitute for an examination, a diagnosis or the advice of your own treating doctor, and it cannot account for your history, medicines or reports. If a symptom is sudden or severe — chest pain, stroke symptoms, breathing difficulty, serious injury, seizures or heavy bleeding — call 9281862028 or come straight to the emergency department instead of reading further.
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