What to do when there's a differential Capillary Blood Glucose (CBG) reading?

Management of Differential Capillary Blood Glucose (CBG) Readings

When encountering discrepant capillary blood glucose readings, immediately verify with a repeat measurement using proper technique, and if the discrepancy persists, confirm with a laboratory venous glucose measurement before making any treatment decisions. 1, 2

Initial Assessment Algorithm

When you encounter differential CBG readings, follow this structured approach:

Step 1: Verify Measurement Technique

• Check for pre-analytical errors first: ensure proper hand hygiene, adequate blood sample volume, and correct strip handling, as these are the most common causes of inaccurate readings 3
• Confirm the meter meets current accuracy standards (95% of readings within 15% for glucose ≥100 mg/dL, or within 15 mg/dL for glucose <100 mg/dL per ISO 15197:2013) 1
• Verify strips are not counterfeit, expired, or from opened/resold vials, as these commonly produce spurious results 1

Step 2: Repeat Measurement Protocol

• Perform an immediate repeat CBG measurement on the same meter to rule out random error 2
• If available, test on a second glucose meter to identify device-specific malfunction 1
• Document both readings and the time interval between measurements 4

Step 3: Clinical Context Evaluation

• Assess whether the reading is consistent with the patient's clinical presentation (symptoms of hypo/hyperglycemia, recent food intake, insulin timing) 1
• Consider potential interferents: high-dose acetaminophen (can cause positive bias in some CGM systems, though less relevant for CBG), extreme hematocrit, hypoxemia, or high-dose vitamin C can affect meter accuracy 1

Decision-Making Framework

If Readings Differ by <15%

• Accept the more recent reading if proper technique was confirmed 1
• Proceed with clinical management based on this value 1
• No additional confirmation needed unless clinically inconsistent 2

If Readings Differ by ≥15% or Are Clinically Inconsistent

• Do not make treatment decisions based on either CBG reading 1, 2
• Obtain laboratory venous glucose measurement as the reference standard 5
• Note that capillary and venous glucose can systematically differ by 5-10%, with comparator methods showing biases up to 8% 5
• Hold insulin dose adjustments until confirmed glucose value is obtained 2

Special Considerations for CGM Users

If the patient uses continuous glucose monitoring and CBG is being performed for confirmation:

• CGM readings should be verified with CBG when there is suspicion of inaccuracy, during rapid glucose changes (>2 mg/dL/min), or when symptoms don't match CGM data 1, 2
• CGM has a 5-15 minute lag time behind blood glucose due to interstitial fluid measurement 6
• "Compression lows" from sleeping on the sensor can cause falsely low CGM readings that require CBG confirmation 2
• CGM accuracy is lowest in hypoglycemic ranges, making CBG verification critical for suspected hypoglycemia 6

Common Pitfalls to Avoid

• Never average two discrepant readings - this approach has no scientific basis and can lead to inappropriate treatment decisions 1
• Don't ignore meter quality variation: only 6 of the top 18 glucose meters met accuracy standards in one surveillance analysis 1
• Avoid using CBG from alternate sites (forearm, thigh) during rapid glucose changes, as these lag behind fingertip measurements 3
• Don't assume all FDA-cleared meters are equally accurate - substantial variation exists among widely used systems 1

Documentation Requirements

• Record both discrepant values with timestamps 4
• Document the action taken (repeat measurement, laboratory confirmation) 4
• Note any identified cause for the discrepancy (technique error, interferent, meter malfunction) 4
• If laboratory confirmation was obtained, document the venous glucose value and how it compared to the CBG readings 5

When to Replace Equipment

• If repeated discrepancies occur with the same meter despite proper technique, replace the device 1
• Ensure replacement meter meets current FDA or ISO accuracy standards 1
• Verify the new meter with control solution before clinical use 3