Which laboratory tests should be ordered to assess diabetes control and screen for end‑organ damage?

Laboratory Tests to Assess Diabetes Control and Screen for End-Organ Damage

To assess diabetes control, order HbA1c every 3 months until glycemic targets are achieved, then every 6 months if stable; to screen for end-organ damage, order annual urine albumin-to-creatinine ratio, serum creatinine with eGFR, lipid panel, and arrange annual dilated eye examination, with more frequent monitoring if abnormalities are detected.

Tests for Assessing Diabetes Control

Hemoglobin A1c (HbA1c)

• HbA1c is the primary test for monitoring long-term glycemic control, reflecting average glucose levels over the preceding 2-3 months 1
• The test should be performed using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) assay 1
• Order HbA1c every 2-3 months when adjusting therapy or if not meeting glycemic goals 2
• Once glycemic targets are stable, testing every 6 months is appropriate 1
• HbA1c has several advantages: no fasting required, greater preanalytical stability, lower within-person variability, and unaffected by recent food intake, stress, or acute illness 1

Self-Monitoring of Blood Glucose (SMBG)

• Patients on multiple daily insulin injections or insulin pump therapy should perform SMBG three or more times daily 1
• For patients on less intensive regimens, SMBG frequency should be individualized based on treatment complexity 1
• Verify patient technique and ability to interpret results at regular intervals 1

Important Caveats for HbA1c Interpretation

• Do not rely on HbA1c alone in conditions with altered erythrocyte turnover: sickle cell disease, hemoglobin variants, severe anemia, recent blood transfusion, hemolysis, glucose-6-phosphate dehydrogenase deficiency, pregnancy, or erythropoietin therapy 1, 3, 4
• In these situations, use fasting plasma glucose or oral glucose tolerance test instead 3, 4
• African Americans may have higher HbA1c levels than non-Hispanic Whites at similar glucose levels, though the association with complications appears similar 1

Tests for Screening End-Organ Damage

Kidney Function (Diabetic Nephropathy)

• Order urine albumin-to-creatinine ratio (UACR) annually to detect early diabetic kidney disease 1
• Measure serum creatinine and calculate estimated glomerular filtration rate (eGFR) annually 1
• If albuminuria is present or eGFR is declining, increase monitoring frequency to every 3-6 months 1

Cardiovascular Risk Assessment

• Order fasting lipid panel (total cholesterol, LDL, HDL, triglycerides) annually 1
• Lipid abnormalities are common in diabetes and contribute significantly to cardiovascular disease risk 1
• More frequent testing may be needed if lipid-lowering therapy is initiated or adjusted 1

Eye Examination (Diabetic Retinopathy)

• Arrange dilated fundoscopic examination by an ophthalmologist or optometrist annually 1
• This is not a laboratory test but is essential for detecting diabetic retinopathy, a specific microvascular complication 1, 5
• More frequent examinations are needed if retinopathy is present 1

Neuropathy Screening

• Perform comprehensive foot examination annually, including assessment of protective sensation using 10-g monofilament testing 1
• While not a laboratory test, this clinical assessment is critical for detecting diabetic neuropathy and preventing foot ulcers 1

Additional Considerations

• Measure blood pressure at every clinical visit to detect hypertension, which accelerates both microvascular and macrovascular complications 1
• Consider thyroid function tests (TSH) in type 1 diabetes or if clinically indicated, as autoimmune thyroid disease is more common in these patients 1

Pitfalls to Avoid

• Do not use point-of-care HbA1c devices for monitoring unless they are FDA-cleared for diagnostic use and performed in CLIA-certified laboratories 1
• Ensure blood samples for glucose measurement are collected in tubes with glycolytic inhibitors and processed promptly; delayed processing causes falsely low glucose values 3
• If marked discordance exists between HbA1c and glucose measurements, suspect assay interference from hemoglobin variants and switch to glucose-based monitoring 3, 4
• Do not overlook the importance of annual screening tests even when HbA1c appears well-controlled, as end-organ damage can progress silently 1, 5