How often are blood tests inaccurate?

Blood tests are a critical diagnostic tool used by doctors to check for disease, monitor chronic conditions, and assess overall health. But like any medical test, they are not 100% accurate. Blood tests can sometimes deliver false positive results, where the test detects something that is not really there, or false negatives, where the test fails to detect something that is present. So how often do inaccurate blood test results happen? And what factors influence the reliability of these laboratory tests?

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How accurate are blood tests overall?

In general, blood tests are highly accurate when performed correctly. Many routine blood tests, like a complete blood count or a basic metabolic panel, have an accuracy rate of over 95%. Other more complex tests, like those looking for specific biomarkers or antigens, may have slightly lower accuracy closer to 90%.

However, it’s impossible to give one accuracy rate for all blood tests. Each test has its own expected rate of false positives and false negatives based on the methodology and purpose of the test. Some newer, less standardized tests tend to have higher rates of inaccuracy compared to older standard tests that have been optimized over decades of use.

Overall though, for most common blood tests, the chance of getting an incorrect result is quite low, less than 5-10%. The higher the quality and specificity of the test, the lower the chance of a false result.

Sources of error in blood tests

There are a few key ways that inaccurate blood test results can occur:

Pre-analytic errors – Issues with how the blood sample was collected, transported, stored or handled prior to testing can alter results.
Analytic errors – Problems with the testing technique, testing equipment or fault in reagents can lead to incorrect analysis.
Post-analytic errors – How the clinician interprets and reports the test results after the fact can also change the accuracy.

Biologic variability – Normal fluctuations or inconsistencies in the measured analyte.
Unsuitable test – The performed test may not be appropriate to accurately measure the desired parameter.

According to studies, a majority of laboratory errors actually fall in the pre-analytic phase. Issues like using the wrong collection tube, inadequate mixing of the blood sample, or delays in transportation can skew the results.

Factors that influence blood test accuracy

There are a number of factors that can affect the occurrence of inaccurate blood test results:

The specific test – Some blood tests are far more susceptible to false results than others. For example, a routine blood glucose test would have lower error rates than a cancer biomarker test looking for very low concentration analytes.
Testing methodology – The technology and exact protocols used to analyze the blood impacts accuracy. Older, manual methods typically have higher errors than automated tests.

Testing personnel – Operator skill and training affects incidence of human errors, especially pre-analytic errors.
Adherence to procedures – Strictly following collection protocols and handling standards minimizes pre-analytic errors.
interference – Certain substances or medical conditions can interfere with test reagents and cause false results.

Sample quality – Heavily hemolyzed, lipemic or contaminated blood specimens are more likely to provide inaccurate results.

Understanding these potential sources of error allow labs to implement better training, standards, automation and quality control to minimize the chances of reporting incorrect blood test results.

How often do false positives happen?

The rate of false positive results varies substantially based on the specific blood test:

HIV blood antibody test – 1 in 5200 false positives
Rapid strep test – 1 in 20 false positives
Rheumatoid arthritis blood test – 1 in 25 false positives

Pregnancy blood test – 1 in 400 false positives
PSA test for prostate cancer – 1 in 7 false positives

As you can see, certain tests like HIV have extremely low false positive rates, while others like PSA have high false positive rates. Tests looking for rare conditions or markers tend to have more false positives.

The consequences of false positives also varies depending on the condition. For example, a falsely positive pregnancy test may cause temporary stress. But a falsely positive HIV test could greatly impact someone’s mental state and relationships.

How often do false negatives occur?

Again, false negative rates differ substantially depending on the specific blood test:

HIV blood antibody test – 1 in 12,500 false negatives

Rapid strep test – 1 in 20 false negatives
Rheumatoid arthritis blood test – 1 in 30 false negatives
Pregnancy blood test – 1 in 500 false negatives

PSA test for prostate cancer – 1 in 20 false negatives

Certain pre-analytic issues like improper sample collection or suboptimal sample handling are contributors to increased false negatives. Disease states can also increase the chances of false negatives for certain tests if the condition itself interferes with the analyte being measured.

Just like false positives, the implications of false negatives can range from inconvenient to devastating. A false negative on a critically important test could delay life-saving treatment or allow a contagious person to inadvertently spread disease.

Can blood tests give false positives due to other conditions?

Yes, many medical conditions or factors unrelated to what’s being tested can cause falsely abnormal blood test results:

Heterophile antibodies – Present in some illnesses, these antibodies interfere with immunoassay tests for cancer or hormone markers, causing false positives.
Rheumatoid factor – High levels can erroneously cause a positive result for certain autoimmune tests.

Elevated bilirubin – Excess bilirubin can interfere with certain spectrophotometric blood assays.
Hemolysis – Ruptured red blood cells release enzymes and iron that can skew multiple blood test results.
Lipemia – Excess lipids in the blood from a recent meal can interfere with some blood chemistry tests.

This type of interf reference is common with qualitative immunoassays and should be considered if a test result is inconsistent with the clinical picture.

Should test results be repeated if the doctor suspects a false result?

If a physician has good reason to doubt the accuracy of an initial blood test result, they should absolutely request follow-up confirmatory testing. Situations where repeat testing may be warranted include:

Test result is highly inconsistent with the patient’s symptoms and clinical presentation.

Screening test was positive, but is known to have a high false positive rate.
Earlier sample had poor quality or collection issues.
Patient has a medical condition or interference present that could skew results.

Testing lab has quality control issues.
Results show drastic or impossible changes from previous tests.

For some tests, like HIV antibody tests, duplicate retesting is automatically done by the lab when initial results are positive. This helps weed out potential false positives before the results get reported out.

In many cases, doing an alternative confirmatory test can help determine the accuracy of questionable blood test results. For instance, following up a positive syphilis screening test with a more specific FTA-ABS test to confirm diagnosis.

Tips to improve accuracy of blood testing

Here are some suggestions to help improve blood test accuracy:

Use reputable labs that follow quality standards and perform result validation.

Ensure proper test tube additives and fill volumes.
Label specimens clearly and completely.
Deliver samples to lab promptly after collection.

Match test requested to clinical situation.
Review factors that could interfere with method.
Consider repeat or alternative testing when appropriate.

Have qualified staff, instrument calibration, and reagent quality control.
Document collection, transport, storage conditions.
Keep open communication between providers and lab.

While total elimination of erroneous blood test results is unlikely, measures like these can help significantly reduce overall rates of inaccuracy.

Common Causes of Erroneous Blood Test Results
Phase	Potential Issue	Example
Pre-analytic	Improper tube additives	Clotted sample gives abnormal results
Pre-analytic	Prolonged tourniquet use	Hemolysis and high potassium level
Pre-analytic	Incorrect order/labeling	Results reported on wrong patient
Analytic	Equipment failure	Erroneous sodium result
Analytic	Improper reagent storage	Unreliable cholesterol level
Post-analytic	Misinterpretation	Low hCG incorrectly read as negative

Conclusion

Blood testing is a critically important and widely used diagnostic tool in modern medicine. While these laboratory tests are generally highly accurate, especially newer automated assays, no test is 100% error-proof. Pre-analytic mistakes in collection or handling remain the most common source of inaccurate results. Certain tests also tend to have higher false positive and negative rates based on the methodology and analyte being measured. When used properly and interpreted within clinical context by trained professionals, blood tests provide vital data to detect and manage disease. Quality control steps help maximize overall reliability, but occasional confirmatory retesting may be needed to verify highly questionable results.