@tatiana987 Medical tests are generally reliable—but they are not infallible, and they are often misunderstood or misused.

1. Are medical lab tests and measurements “unreliable”?

In general, no.
Modern medical testing (blood labs, imaging, blood pressure measurements, vascular studies, etc.) is built on decades of validation, quality control, and standardization.
That said, "reliability is not the same as perfection".
Most common lab tests are highly reproducible *when done correctly.*
Imaging and physiologic tests are usually accurate within known limits.
Errors are uncommon, but *interpretation errors* are more frequent than technical errors.

2. Where problems actually arise (most common issues):

A. Biological variability (normal human fluctuation):

Many values change naturally:

Blood pressure varies minute to minute.

Cholesterol and glucose fluctuate day to day.

Creatinine and eGFR vary with hydration, illness, medications.

A single abnormal result does not always equal disease.

B. Pre-analytical factors (very important):

Results can be affected by:

✓Fasting vs non-fasting
✓Dehydration
✓Recent exercise
✓Acute illness or inflammation
✓Timing of the test
✓Improper cuff size for blood pressure

Lab handling issues *(rare, but possible)*----
These factors can skew results without the lab itself being “wrong.”

C. Reference ranges are statistical—not personal.

“Normal ranges”:

✓Are based on large populations
✓Do not account for individual baselines
✓May not reflect what is normal for you

Someone can be:

✓“In range” and still unwell
✓“Out of range” and clinically fine
**This is where clinical judgment matters.

D. Interpretation errors *(more common than lab errors)*
✓Tests do not diagnose people—clinicians interpret them.

Problems occur when:

✓Results are taken out of context
✓Trends over time are ignored
✓Symptoms are dismissed because “labs look okay”
✓Borderline abnormalities are over- or under-reacted to
✓This is often what patients experience as “the system failing.”

3. How accurate are specific types of tests?

✓Blood tests generally very accurate
✓Most errors come from interpretation, not measurement
✓Blood pressure Is accurate only if *measured correctly*
✓One reading in a clinic is often misleading
✓Home or ambulatory monitoring is often more reliable
✓Imaging (CT, MRI, ultrasound) is technically precise

Findings can be:

✓Overcalled
✓Undercalled
✓Clinically insignificant but alarming
Vascular and cardiac studies are usually reliable
✓False positives and false negatives do occur
✓Best interpreted alongside symptoms and risk factors

4. Should you “trust” medical tests?

A healthier mindset is this:

✓Trust medical tests as useful tools—not as absolute truth.

They are best used to:

✓Identify trends
✓Support or rule out hypotheses
✓Guide further evaluation
✓Monitor change over time

They are not crystal balls and should not replace clinical reasoning or patient experience.

Here are links to credible sources that support the above information:

1) Medical labs and variability in test results

Biological:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9200014/ �
PMC

“Preanalytical Variables:
https://australianprescriber.tg.org.au/articles/preparation-for-blood-tests-what-can-go-wrong-before-the-sample-reaches-the-lab.html �
Australian Prescriber

2) Blood pressure measurement accuracy and variability:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10453092/ �
PMC

American Heart Association:

https://www.aafp.org/pubs/afp/issues/2021/0900/p237.html �

https://www.heart.org/en/health-topics/high-blood-pressure/understanding-blood-pressure-readings/monitoring-your-blood-pressure-at-home �
http://www.heart.org

Wikipedia overview:
https://en.wikipedia.org/wiki/Blood_pressure_measurement �
Wikipedia

3) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9200014/ �
PMC

https://academic.oup.com/clinchem/article/61/7/914/5611492 �

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10453092/ �
PMC

https://www.aafp.org/pubs/afp/issues/2021/0900/p237.html �