Which Tests to Order Alongside a Continuous Glucose Monitor (CGM)

What a CGM Actually Measures and Where It Falls Short

A continuous glucose monitor samples interstitial glucose every 1 to 5 minutes, generating roughly 288 readings per day. That density reveals patterns no single fasting glucose draw can capture: postprandial spikes, overnight hypoglycemia, and the dawn phenomenon. The 2023 ADA Standards of Care define time in range (TIR) as the percentage of readings between 70 and 180 mg/dL, with a target of ≥70% for most adults with diabetes.

But CGM has blind spots. It cannot tell you whether insulin resistance is driving those spikes, whether beta-cell function is declining, or whether cardiovascular risk is climbing alongside glucose. A 2022 Endocrine Society clinical practice guideline on CGM use noted that CGM data should be "interpreted in the context of other metabolic parameters." That context comes from paired labs.

Think of CGM as a dashcam. It records what happens on the road. The labs listed below are the engine diagnostics that explain why.

HbA1c: The 90-Day Average That Validates Your CGM

HbA1c reflects glycated hemoglobin over the preceding 2 to 3 months, making it the standard comparator for CGM-derived glucose management indicator (GMI). When GMI and HbA1c diverge by more than 0.5%, clinicians should investigate causes such as hemoglobin variants, iron deficiency anemia, or chronic kidney disease, all of which skew HbA1c independently of true glucose exposure [1].

The ADA recommends an HbA1c target of <7.0% for most non-pregnant adults with diabetes. In the landmark DCCT trial (N=1,441), each 1% reduction in HbA1c lowered microvascular complication risk by approximately 35% [2]. Ordering HbA1c at baseline and every 3 months alongside CGM lets you confirm that sensor-derived metrics match laboratory reality.

Ordering tip: draw HbA1c the same week you download CGM data so the comparison window aligns.

Fasting Insulin and HOMA-IR: Catching Resistance Early

Glucose can stay normal for years while insulin climbs to compensate. A fasting insulin level above 10 to 15 µIU/mL, combined with fasting glucose, generates a HOMA-IR score. A HOMA-IR above 2.0 suggests insulin resistance; above 2.9 strongly indicates it [3]. The AACE 2023 consensus statement on insulin resistance identifies HOMA-IR as a practical screening tool in primary care.

Why this matters for CGM users: a person with "normal" TIR but a HOMA-IR of 3.5 is producing double the insulin needed to keep glucose flat. CGM will not flag that. Their beta cells are working overtime, and without this lab, the trajectory toward type 2 diabetes stays invisible.

Pair fasting insulin with fasting glucose drawn after a 10-to-14-hour fast, ideally on a morning when CGM data shows stable overnight glucose.

C-Peptide: Measuring What Your Pancreas Is Actually Making

C-peptide is released in equimolar amounts with insulin from pancreatic beta cells. It has a longer half-life (approximately 30 minutes vs. 4 to 6 minutes for insulin), making it a more stable marker of endogenous production [4]. For patients on exogenous insulin, C-peptide is the only reliable way to assess residual beta-cell function because exogenous insulin does not contain C-peptide.

The Endocrine Society and ADA both reference C-peptide when classifying diabetes type and guiding therapy transitions. A fasting C-peptide below 0.6 ng/mL generally indicates significant beta-cell loss. A value above 1.0 ng/mL with hyperglycemia points toward insulin resistance rather than insulin deficiency.

Clinical scenario: a CGM shows persistent postprandial glucose above 200 mg/dL. If C-peptide is 0.3 ng/mL, the patient likely needs basal-bolus insulin. If C-peptide is 3.2 ng/mL, the problem is resistance, and GLP-1 receptor agonists, metformin, or lifestyle changes become first-line.

Lipid Panel and ApoB: The Cardiovascular Risk CGM Cannot See

Dysglycemia and dyslipidemia travel together. Among adults with type 2 diabetes, cardiovascular disease remains the leading cause of death, accounting for roughly 50% of mortality in this population according to the American Heart Association [5]. A standard lipid panel (total cholesterol, LDL-C, HDL-C, triglycerides) is the minimum. ApoB adds precision because it counts all atherogenic particles, not just cholesterol mass.

The 2019 ACC/AHA guideline recommends fasting lipids at least every 5 years for average-risk adults and more frequently for those with diabetes or metabolic syndrome. For CGM users, ordering lipids at baseline and every 6 to 12 months creates a parallel metabolic timeline.

Triglyceride-to-HDL ratio deserves attention. A ratio above 3.0 correlates with small dense LDL particles and insulin resistance. Watching this ratio alongside CGM trends provides a composite metabolic signal that neither test delivers alone.

Liver Enzymes (ALT, AST) and MASLD Screening

Metabolic dysfunction-associated steatotic liver disease affects roughly 30% of the global adult population, and prevalence rises to 55 to 70% in people with type 2 diabetes according to a 2023 meta-analysis published in The Lancet Gastroenterology & Hepatology [6]. Elevated ALT (above 30 U/L in men, above 19 U/L in women by updated thresholds) is a simple screening signal.

CGM data showing frequent hyperglycemic excursions, especially postprandial spikes above 180 mg/dL, may correlate with hepatic insulin resistance and steatosis. But CGM cannot image your liver. ALT and AST, along with a FIB-4 index calculated from age, AST, ALT, and platelet count, fill that gap. The AACE recommends MASLD screening in all patients with obesity or type 2 diabetes.

A normal CGM profile does not rule out liver disease. Some patients maintain glucose homeostasis through hyperinsulinemia while their liver accumulates fat.

hsCRP and Inflammatory Markers

High-sensitivity C-reactive protein (hsCRP) measures low-grade systemic inflammation. Values above 3.0 mg/L are associated with higher cardiovascular event risk, independent of lipid levels, according to the JUPITER trial (N=17,802) [7]. Inflammation also drives glycemic variability: proinflammatory cytokines impair insulin signaling at the receptor level.

For CGM users with unexplained glucose variability, particularly those with autoimmune conditions, chronic infections, or obesity, hsCRP helps distinguish metabolic inflammation from acute illness. A persistently elevated hsCRP alongside erratic CGM tracings suggests that addressing the inflammatory driver (weight loss, infection treatment, dietary modification) may improve glucose patterns more than adjusting diabetes medications alone.

Dr. Anne Peters, professor of medicine at the Keck School of Medicine of USC, has stated: "CGM tells you the 'what.' The labs tell you the 'why.' You need both to make decisions that actually change outcomes."

Thyroid Panel: An Overlooked Driver of Glucose Patterns

Thyroid dysfunction alters glucose metabolism from multiple angles. Hyperthyroidism accelerates hepatic gluconeogenesis and intestinal glucose absorption, producing CGM patterns that mimic poor dietary control. Hypothyroidism slows insulin clearance and can cause unexplained hypoglycemia or blunted postprandial responses [8]. A 2020 review in the European Journal of Endocrinology confirmed bidirectional interactions between thyroid hormones and glucose homeostasis.

Order TSH and free T4 at baseline. If TSH is abnormal, add free T3 and thyroid peroxidase antibodies. Autoimmune thyroid disease and type 1 diabetes coexist in 15 to 30% of cases, per the Endocrine Society, making this combination especially relevant for CGM users with type 1.

Kidney Function: eGFR and Urine Albumin-to-Creatinine Ratio

The kidneys filter roughly 180 grams of glucose per day. When estimated glomerular filtration rate (eGFR) drops below 60 mL/min/1.73 m², glucose handling changes. SGLT2 inhibitors lose efficacy. HbA1c becomes less reliable because of altered red blood cell turnover. CGM actually becomes more important in chronic kidney disease (CKD) stages 3 to 5, but it must be paired with eGFR and urine albumin-to-creatinine ratio (UACR) to contextualize the readings [9].

The ADA Standards of Care recommend annual UACR screening and eGFR measurement for all adults with diabetes. UACR above 30 mg/g indicates microalbuminuria, the earliest detectable sign of diabetic nephropathy. This test is inexpensive (typically under $20 with insurance) and changes management directly: ACE inhibitors or ARBs are indicated, and SGLT2 inhibitors carry renoprotective benefits in this population.

Comprehensive Metabolic Panel: The Baseline Package

A CMP captures sodium, potassium, chloride, bicarbonate, BUN, creatinine, glucose, calcium, total protein, albumin, bilirubin, alkaline phosphatase, AST, and ALT in a single draw. It overlaps with several individual tests listed above but offers them as an efficient, low-cost bundle.

For CGM users, the CMP provides electrolyte status (relevant for patients on metformin or diuretics), albumin (a nutritional marker in patients with significant weight loss on GLP-1 agonists), and a baseline glucose reading for same-day CGM calibration.

Order a CMP as your foundational panel. Then add the specialized tests (fasting insulin, C-peptide, hsCRP, lipid panel with ApoB, thyroid panel, UACR) based on clinical suspicion and CGM patterns.

How to Time Your Lab Draw With CGM Data

Timing matters. Draw fasting labs after a 10-to-14-hour overnight fast, during a period when CGM data shows stable glucose (no meal within 4 hours, no exercise within 2 hours). Schedule the blood draw for morning, ideally between 7:00 and 9:00 AM, to align with cortisol rhythms that affect fasting glucose.

Download CGM data the same week as the lab draw. Overlay the ambulatory glucose profile (AGP) report with lab results in a single visit note. This approach lets clinicians see, for example, that a patient with an HbA1c of 6.8%, a HOMA-IR of 3.1, and a triglyceride-to-HDL ratio of 4.2 has insulin resistance driving their modest hyperglycemia, not beta-cell failure.

The 2022 international consensus on TIR (Battelino et al.) recommends reviewing CGM metrics alongside lab data at every diabetes visit, at minimum every 3 months [10].

Building Your Paired-Test Order Set

For a first-time CGM user with suspected or confirmed metabolic dysfunction, a practical order set includes:

Tier 1 (order for every CGM user):

• HbA1c
• Comprehensive metabolic panel (CMP)
• Fasting insulin
• Lipid panel with calculated LDL

Tier 2 (add based on clinical context):

• C-peptide (if diabetes type is unclear, or patient is on insulin)
• ApoB (if family history of premature cardiovascular disease, or triglycerides above 150 mg/dL)
• hsCRP (if obesity, autoimmune disease, or unexplained glycemic variability)
• TSH and free T4 (if symptoms of thyroid dysfunction, or type 1 diabetes)
• UACR (if diabetes duration exceeds 5 years, or hypertension is present)

Tier 3 (specialist-directed):

• GAD65 and IA-2 antibodies (if late-onset autoimmune diabetes suspected)
• Cortisol (AM draw; if Cushing syndrome is in the differential)
• IGF-1 (if acromegaly is suspected based on CGM patterns showing persistent hyperglycemia with low insulin)

Recheck Tier 1 labs every 3 months for the first year, then every 6 months if glucose targets are met. Adjust Tier 2 frequency based on clinical trajectory.

As the AACE 2023 comprehensive diabetes management algorithm states: "Glycemic metrics should never be interpreted in isolation. Lipids, hepatic function, renal function, and inflammatory markers form the metabolic context without which glucose data are incomplete."

Fasting insulin below 5 µIU/mL with stable CGM TIR above 85% and an HbA1c below 5.7% represents optimal metabolic health by current evidence. That is the target these paired tests are designed to track.