CDCES

Laboratory data establish the baseline, identify complications, and show whether the plan is working. The specialist does not order most tests but must know what each value means, when it is due, and when a result is unreliable, because exam questions frequently present lab values that conflict with the person's glucose logs or CGM data.

Hemoglobin A1C

Hemoglobin A1C reflects average glycemia over the preceding 2 to 3 months (weighted toward the most recent 30 days) because glucose binds irreversibly to hemoglobin for the life of the red blood cell (about 120 days). The goal is <7% for many nonpregnant adults, individualized for age, comorbidities, hypoglycemia risk, and life expectancy. Less stringent targets (such as <8%) suit older adults with complex health, limited life expectancy, or high hypoglycemia risk. More stringent targets (such as <6.5%) may suit younger people and those planning pregnancy when achievable without significant hypoglycemia. A1C is checked at least twice a year in people meeting goals and quarterly when therapy has changed or goals are not being met.

eAG (mg/dL) = 28.7 × A1C − 46.7

A1C 6% ≈ 126 mg/dL · 7% ≈ 154 mg/dL · 8% ≈ 183 mg/dL · 9% ≈ 212 mg/dL. Each 1% change in A1C ≈ 29 mg/dL change in average glucose.

A1C is only as accurate as the red blood cells it measures. Any condition that shortens or lengthens red cell survival, or that alters the hemoglobin molecule, makes A1C misleading. When A1C and glucose data disagree, the specialist looks for one of these conditions and uses an alternative measure.

Table1.5—Conditionsthatlimita1Caccuracy

Condition	EffectonA1C	Mechanismandnotes
Hemolyticanemia acuteblood ,	Falselylow	Youngerredcellpopulationhaslesstimetoglycate;transfused
loss recenttransfusion ,		cellsdilutetheperson'sownhemoglobin
CKD on erythropoiesis- stimulatingagents(ESA) , hemodialysis Pregnancy(secondandthird trimesters)	Falselylow Falselylow	ESAtherapyproducesnewredcells;A1Ccorrelatespoorlywith 1 glucoseinadvancedCKDanddialysis Increasedredcellturnover;glucosemonitoringisprimary A1C , secondary(target<6%ifachievablewithouthypoglycemia)

Table1.5—Conditionsthatlimita1Caccuracy(Continued)

Condition	EffectonA1C	Mechanismandnotes
Iron-deficiency anemia	Falselyhigh	Olderredcellpopulation;A1Cmayfallafterironreplacement
(untreated)
Hemoglobinopathies(sicklecell	Variable	Interfereswithsomeassaysandaltersredcellsurvival;usean
disease HbC HbE thalassemia) , , ,		assay without interference or a non-A1C measure Sickle cell trait .
		isassociatedwithslightlylowerA1Catthesameglucoselevel
Recentrapidglucosechange(new	Lagsbehind	A1Creflectsmonths;itcanbeonlymildlyelevatedwhenglucose
type 1 diabetes steroid -induced ,		hasrisenquickly
hyperglycemia)

Table1.6—Alternativesandcomplementstoa1C

Measure	Timeframe	Use
Fructosamine(glycated	About2–3weeks	WhenA1Cisunreliable(hemoglobinopathy anemia pregnancy)orto , ,
serumproteins)		seeanearlyresponsetoatreatmentchange;affectedbylowalbumin
CGMmetrics(timeinrange ,	Atleast14dayswith≥70%	Time in range 70–180 mg/dL (goal >70%) time below range <70 (goal ,
GMI)	wear	<4%)and<54(<1%);glucosemanagementindicator(GMI)estimates
		A1Cfrommeansensorglucose;showshypoglycemiaandvariability
		thatA1Chides
Fastingglucose	Singletimepoint	Reflectsovernighthepaticglucoseoutputandbasalinsulinadequacy;
		target80–130mg/dLbeforemeals
Postprandialglucose	1–2hafterstartofmeal	Reflectsthemealresponseandmealtimemedication;targetpeak<180
		mg/dL;usefulwhenA1Cisabovegoaldespitefastingvaluesattarget

A1C tells you the average; it does not show hypoglycemia or variability. Two people with the same A1C of 7% can have very different time in range: one steady, one swinging between 50 and 300 mg/dL. When A1C looks "at goal" but the person reports frequent lows, CGM or structured glucose monitoring is the next assessment step.

Lipids, Kidney, Thyroid, and Other Tests

The lipid panel assesses cardiovascular risk. LDL cholesterol is the primary target of therapy; triglycerides are often elevated and HDL cholesterol low when glycemia is poorly managed. Statin intensity and LDL goals depend on age and cardiovascular risk: moderate-intensity statin at minimum for adults 40–75 years without cardiovascular disease, and high-intensity statin with an LDL goal of <55 mg/dL for those with atherosclerotic cardiovascular disease (ASCVD). Triglycerides ≥500 mg/dL call for evaluation of secondary causes and treatment to reduce pancreatitis risk.

Kidney function assessment includes serum creatinine with estimated glomerular filtration rate (eGFR) and the urine albumin-to-creatinine ratio (UACR) on a spot urine sample. Screening is annual, beginning at diagnosis for type 2 diabetes and 5 years after diagnosis for type 1 diabetes. Because albumin excretion varies (exercise, fever, infection, menstruation, marked hyperglycemia, or hypertension can raise it temporarily), 2 of 3 samples over 3–6 months must be elevated to confirm albuminuria. Declining kidney function changes medication selection and dosing: metformin is contraindicated below eGFR 30 and not started between 30 and 45; SGLT2 inhibitors are used for kidney and heart protection down to eGFR 20; insulin and sulfonylurea clearance falls, raising hypoglycemia risk.

Table1.7—Kidneyscreeningcategories

Measure	Category	Interpretation
UACR<30mg/g	Normaltomildlyincreased	Rescreenannually
UACR30–299mg/g	Moderatelyincreasedalbuminuria	Earlykidneydamageandamarkerofcardiovascularrisk;
		optimizeBPandglucose ACEinhibitororARB SGLT2 , ,
		inhibitor;monitormoreoften
UACR≥300mg/g	Severelyincreasedalbuminuria	Higherriskofprogression;addtherapiessuchas
		finerenoneperthecareteam;monitorUACRandeGFRup
		to4timesayear
eGFR≥60	Normalormildlyreduced	CKDisstillpresentifalbuminuriaispersistent
eGFR<60	Reducedkidneyfunction(CKDstage	Reviewmedicationdoses;eGFR<30warrants
	3orworse)	nephrologyreferral

Thyroid function (TSH) testing is recommended for people with type 1 diabetes because of the increased risk of autoimmune thyroid disease: soon after diagnosis, then periodically (every 1–2 years) or when symptoms appear. Thyroid dysfunction affects weight, metabolism, and glycemia; hyperthyroidism worsens hyperglycemia, and hypothyroidism can increase hypoglycemia risk and raise LDL. Celiac disease is also more common in type 1 diabetes: screen with tissue transglutaminase IgA (tTG-IgA, with total IgA) soon after diagnosis, repeat within 2 years and again at 5 years in children, and test anyone with diarrhea, abdominal pain, poor growth, weight loss, or unexplained hypoglycemia. People taking metformin long term should have vitamin B12 measured periodically, especially with peripheral neuropathy or anemia, because deficiency can cause or worsen neuropathy.

Table1.8—Laboratoryandscreeningschedule

Test	When	Notes
A1C	Atleastevery6monthsifmeetinggoals;every3	Individualizedgoal(<7%formanyadults)
	monthsiftherapychangedornotmeetinggoals
Lipidpanel	Atdiagnosisorinitialevaluation;4–12weeks	LDLprimarytarget
	afterstartingorchangingstatin thenannually; ,
	every5yearsifunder40andnotontherapy
UACRandeGFR	Annually:type2atdiagnosis;type1after5	Confirmalbuminuriawith2of3samples
	years'duration;moreoftenwithCKD
LiverenzymesandFIB-4	Type2diabetesorprediabeteswith	FIB-4 (age AST ALT platelets) screens for , , ,	liver
	cardiometabolicrisk	fibrosis from metabolic dysfunction–associated
		steatoticliverdisease
TSH	Type 1: soon after diagnosis then every 1–2 ,	Autoimmunethyroiddiseaserisk
	yearsorwithsymptoms
Celiacscreen(tTG-IgA)	Type1:soonafterdiagnosis;repeatinchildren	MeasuretotalIgAtoavoidfalsenegatives
	within2yearsandat5years;anytimewith
VitaminB12 Islet autoantibodies C- , peptide	symptoms Periodically with long-term metformin Whendiabetestypeisuncertain	1 Especiallywithneuropathyoranemia Autoantibodies (GAD65 IA-2 ZnT8 insulin) support , , , type 1; low C -peptide shows insulin deficiency

Table1.8—Laboratoryandscreeningschedule(Continued)

Test	When	Notes
Dilatedeyeexam	Type2atdiagnosis;type1within5yearsof	Retinalphotographywithremotereadingisan
	onset; then every 1–2 years if no retinopathy at ,	acceptablescreeningmethod
	leastannuallyifpresent
Comprehensivefootexam	Atleastannually;inspectioneveryvisitifathigh	Monofilamentplusoneothertest
	risk

A person with type 1 diabetes who develops unexplained hypoglycemia, falling insulin needs, weight loss, or GI symptoms should be evaluated for celiac disease (malabsorption) and, less commonly, adrenal insufficiency or hypothyroidism. A person on long-term metformin with new numbness in the feet may have B12 deficiency rather than, or in addition to, diabetic neuropathy.

A1C reflects 2–3 months of glycemia; goal <7% for many

UACR and eGFR annually: type 2 at diagnosis, type 1 after 5

adults, <8% for older adults with complex health or high

years; albuminuria is UACR ≥30 mg/g in 2 of 3 samples.

hypoglycemia risk.

Metformin: contraindicated at eGFR <30; check vitamin B12

A1C is falsely low with hemolysis, blood loss, transfusion,

periodically with long-term use.

ESA therapy, dialysis, and later pregnancy; falsely high with

Type 1 diabetes: screen for thyroid disease (TSH) and celiac

untreated iron deficiency.

disease (tTG-IgA) soon after diagnosis and periodically.

Use fructosamine (2–3 weeks) or CGM metrics when A1C is

LDL cholesterol is the primary lipid target; high-intensity

unreliable; CGM needs ≥14 days with ≥70% wear.

statin with LDL <55 mg/dL for people with ASCVD.