Blood tests for patients on low carb
As you go through this guide, keep in mind that many of the normal reference ranges were developed on populations eating a high carb diet, and therefore in some cases you may need to adjust your concept of normal for a patient following a low-carb lifestyle. In addition, many normal values vary based on the individual lab, the patients gender, size, etc. We try to clarify where these caveats are most important.
- Alanine aminotransferase (ALT)
- Fasting blood glucose
- Hemoglobin A1c (HbA1c)
- Complete lipid panel
- Creatinine
- Microalbumin/creatinine ratio, urine
- Optional blood tests
- Complete blood count (CBC)
- Electrolytes (sodium and potassium)
- Fasting insulin or c-peptide
- High-sensitivity c-reactive protein (hs-CRP)
- Nuclear magnetic resonance (NMR) lipoprofile
- Thyroid stimulating hormone (TSH)
- Uric acid
- Vitamin B12
Alanine aminotransferase (ALT)
ALT is a liver enzyme test measuring hepatocellular damage. It can often be elevated in metabolic syndrome and obesity most commonly due to non-alcoholic fatty liver disease (NAFLD). Excessive alcohol intake and liver infections such as hepatitis can also cause elevations.
When to check
Baseline. If the baseline result is abnormal, repeat every 3-6 months.
Standard reference ranges
Men: 10-33 IU/L
Low-carb considerations
Elevated ALT due to metabolic syndrome or NAFLD tends to improve on a low carb diet, at least longer term.
Occasionally, ALT might increase during the initial three months on starting a low-carb diet, coinciding with weight loss. Once the weight loss is over, this elevation tends to disappear and ALT usually normalizes.
Fasting glucose
Fasting blood glucose (FBG) is most commonly used to diagnose or assess diabetes, monitor hyperglycemia and hypoglycemia, and aid in diabetes treatment decisions.
When to check
Baseline and annually at a minimum. However, many may find value checking multiple times per day or even continuously with a continuous glucose monitor (CGM). This allows real time feedback as to the effect food and other lifestyle choices have on blood sugar.
Standard reference ranges
On low carb: 54 – 100 mg/dL (or higher, see below)
Elevated levels
Higher than normal values may be caused by:
- Type 1 or type 2 diabetes
- Acromegaly
- Acute/chronic pancreatitis
- Cushing’s syndrome
- Drug side effects: glucocorticoids
- Pheochromocytoma
- Stress
Decreased levels
Lower than normal values may be caused by:
- Adrenal insufficiency
- Alcohol consumption
- Exogenous insulin
- Hypopituitarism
- Hypothyroidism
- Insulinoma
- Malignancy
- Malnutrition
- Drug side effects, e.g oral hypoglycemic agents
- Sepsis
Low-carb considerations
Patients eating low carb may have much lower FBG levels than patients following the traditional food pyramid.
However, FBG numbers can also go up on low carb, which can surprise patients and doctors alike. This common occurrence is the “dawn phenomenon” and reflects that muscles are in “glucose refusal mode” — or what some low carb doctors have dubbed “adaptive glucose sparing.” Generally, this is not a concern as the rest of the day blood glucose will remain low.
- Is your fasting blood glucose higher on low carb or keto? Five things to know
- The dawn phenomenon – why are blood sugars high in the morning?
Hemoglobin A1c (HbA1c)
Hemoglobin A1c (HBA1c) estimates the average glucose level in the blood over the previous three months by counting the number of glucose molecules stuck on red blood cells. Glucose molecules on hemoglobin is called glycated, or glycosated hemoglobin.
When to check
Baseline. If abnormal, repeat every 3 months.
Standard reference ranges
Normal values: 4.0 to 5.6%
Pre-diabetes: 5.7 to 6.4%
Diabetes: ≥ 6.5%
Elevated values
Higher than normal values may be caused by:
- Diabetes (type 1 and 2)
- Pre-diabetes
- Low red blood cell (RBC) turn over, e.g. anemia of iron deficiency
- B12 deficiency, folate deficiency
Decreased values
Lower than normal values may be caused by:
- High RBC turn over, e.g.hemolytic anemia
- Use or abuse of erythropoietin (EPO)
- Treatment for iron deficiency, B12 deficiency, or folate deficiency
- Patients on hemodialysis (which alters the RBC turn over)
Low-carb considerations
Ordering HbA1c tests every three months can motivate and empower patients to continue the low carb diet.
Some patients have HbA1c levels that stall or even rise at the start of low-carb, especially as they reduce their diabetic medications. Let them know this is okay.
Example: a patient has a baseline HbA1c of 7.4%, but daily was taking 100 units of long acting insulin, a high dosage of a sulfonylurea, and an SGLT2-inhibitor. After going low-carb, the patient was able to discontinue all her diabetic medications within six months. Her six month HbA1c level is still 7.4%. The patient feels frustrated and confused that her HbA1c did not go down despite all her hard work.
This is a common situation in low-carb clinics.
More
HbA1c results may not be accurate for patients with chronic kidney disease (CKD), underestimating true blood glucose values. Results may also be inaccurate with some genetic hemoglobin variants, such as persistence of fetal hemoglobin (HbF) or existence of sickle cell anemia, either measuring too high or too low against real blood glucose values.
Complete lipid panel
The complete lipid panel measures total cholesterol, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C) and triglycerides. It is primarily used to assess cardiac risk and make decisions regarding treatment for patients who appear to be at high-risk.
When to check
Baseline. Repeat every 6 months. If a patient appears to be a hyper-responder — having LDL-C levels significantly higher on low carb — consider repeating every 3 months until stable, then repeat every 6 months.
Standard reference ranges
Total cholesterolLow-carb considerations
Patients should fast for 12 hours prior to a lab test for a complete lipid panel as eating can cause large fluctuations in triglyceride levels. Keep in mind the “normal” values were likely determined on people eating high carb diets. Most patients eating low carb will have much lower triglyceride levels. Therefore, low carb doctors consider the optimal triglyceride levels for patients on low-carb to be less than 100mg/dL and close to 70 mg/dL (<0.8 mmol/L).
Most studies show that patients eating low carb do not see a significant increase in LDL, and instead see a decrease in TG and increase in HDL.
For an estimated 5 to 25% of people – whether weight loss occurs or not – LDL cholesterol can go up significantly in response to very-low-carb diets, sometimes by 200% or more.
Creatinine
The creatinine test is a clinical marker for kidney function. Serum levels of creatinine, a waste product from the normal wear and tear of muscles, will vary depending on patients’ muscle mass, age, sex, and their glomerular filtration rate (GFR). Therefore, normal reference ranges may also vary depending on these variables.
When to check
Baseline. Repeat every 6 months if abnormal or the patient is known to have CKD.
Standard reference ranges
Increased values
Higher than normal values may be caused by:
- Acute tubular necrosis
- Drug side effects, e.g. aminoglycoside antibiotics
- Congestive heart failure
- Dehydration
- Diabetic nephropathy
- Glomerulonephritis
- Renal failure
- Hypothyroidism
- Muscular dystrophy
- Pyelonephritis
- Rhabdomyolysis
- Shock
- Urinary track obstruction
Decreased values
Lower than normal values may be caused by:
- Decreased muscle mass
- Myasthenia gravis
- Muscular dystrophy
Low-carb considerations
Low-carb patients lose excess water weight in the first weeks to months of adopting their new diet.
Some may have concern that low-carb or keto diets are potentially harmful for kidney function. This however, is a myth not supported by scientific evidence. We cover this thoroughly in our guide to low-carb and kidney function.
Microalbumin/creatinine ratio, urine
Microalbumin/creatinine ratio (urine) is commonly used as a marker to detect early diabetic nephropathy. Also known as microalbumin, ACR, UACR, urine albumin, the test measures when kidneys leak small amounts of albumin into the urine. Microalbuminuria is diagnosed either from a 24-h urine collection or from elevated concentration in a spot urine sample. Both must be measured on at least two of three measurements over a two to three-month period.
When to check
Baseline. If abnormal or the patient has CKD, repeat every 6 months. Once levels are stable, repeat annually.
Standard reference ranges
Increased values
Higher than normal values may be caused by:
- Chronic kidney disease (CKD)
- Cardiovascular disease
- Type 1 or type 2 diabetes
- Diabetic nephropathy
- Hypertension
- Vascular endothelial dysfunction
Low-carb reference considerations
There is not much data on improvement of albumin to creatinine ration on low-carb diets, but anecdotal experience and a published case report does suggest this may occur.
Optional blood tests
Depending on other medical conditions your patients may have — such as chronic kidney disease or hypothyroidism — you may want to order other tests. Patients may also struggle with the induction or keto flu when transitioning to a low-carb diet. Certain tests, such as sodium and vitamin B12, may help rule out any other factors that may be causing their symptoms.
Complete blood count (CBC)
CBC (complete blood count) measures the red, white and platelet blood cells in the blood. Red blood cells (erythrocytes) deliver oxygen while white blood cells (leucocytes) are part of the immune system, and platelets (thrombocytes) help in the clotting process. Red blood cells live approximately 120 days, or 3 months, which is why the HbA1c is measured at least 3 months apart.
When to check
If your patient is struggling with fatigue, appears pale, or has any bleeding issues.
Standard reference ranges
WBCs (white blood cells)Men: 5 million-6 million cells/mcL
Men: 14-17 gm/dL
Men: between 41% and 50%
Increased values
Higher than normal values may be caused by:
- Congenital heart disease
- Dehydration
- High altitude
- Hypoxia (such as from undiagnosed sleep apnea)
- Malignancy
- Polycythemia vera
- Pulmonary fibrosis
- Thalassemia
Decreased values
Lower than normal values may be caused by:
- Anemia
- Chronic renal failure
- Alcohol dependence
- Drug side effect, e.g. allopurinol
- Liver disease
Low-carb considerations
Checking CBC may be helpful if your patient is experiencing unexplained fatigue or mental fog. These symptoms can also occur if the patient has a sub-optimal intake of protein, calories or salt, or has the induction or keto-flu.
Electrolytes (sodium and potassium)
Sodium
Testing for sodium levels can help troubleshoot concerning symptoms, such as fatigue, muscle cramps and headache on a low carb diet. Sodium is an essential electrolyte that helps with blood volume regulation, blood pressure, fluid balance, nerve induction, muscle contraction, maintenance of osmotic equilibrium, and acid-base balance.
When to check
Baseline. Repeat every 6 months for patients with CKD
Standard reference ranges
Increased values
Higher than normal values may be caused by:
- Cushing’s syndrome/disease
- Dehydration
- Too high dietary sodium intake
- Type 1 or type 2 diabetes
- Diabetes insipidus
- Excessive diaphoresis
- Advanced age
- Hyperaldosteronism
- Oral contraceptive pills
- Vomiting
Decreased values
Lower than normal values may be caused by:
- Drug side effects: e.g. ACE inhibitors, thiazide diuretics
- Acute tubular necrosis
- Adrenal insufficiency
- Congestive heart failure
- Cirrhosis
- Diarrhea
- Excessive exercise
- Heavy sweating
- Hypothyroidism
- Malnutrition
- Nephrotic syndrome
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Vomiting
- Water intoxication
- Too low dietary sodium intake
Low-carb considerations
Sometimes healthy patients and even renal patients, despite their CKD, must supplement with salt when adopting a low-carb diet.
Fasting insulin or c-peptide
Testing fasting insulin, or c-peptide, can provide insight into a patient’s degree of insulin production, insulin sensitivity or insulin resistance.
Both tests assess the body’s creation and use of insulin, a hormone secreted by the isles of Langerhans in the pancreas. Central to regulating carbohydrates and fat metabolism in the body, insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood. Insulin resistance is the underlying cause of type 2 diabetes.
A fasting insulin level along with a fasting glucose can be used to calculate a HOMA-IR, which provides a fairly accurate assessment of metabolic syndrome, and likely better than simply fasting blood glucose or HbA1c alone.
Traditionally, a c-peptide test is measured as a means of distinguishing between type 1 and type 2 diabetes.
When to check
Baseline and every 6 months
Standard reference ranges
Fasting c-peptide levels: 0.78-1.89 mg/mL
Increased fasting insulin
Higher than normal values may be caused by:
- Acromegaly
- Cushing’s syndrome
- Type 2 diabetes
- Drug side effects, e.g. glucocorticoids, hypoglycemic agents, insulin (exogenous)
- Liver disease
- Insulinoma
- Obesity
- Uremia
Decreased fasting insulin
Lower than normal values may be caused by:
- chronic pancreatitis
- Type 1 diabetes
- Hypopituitarism.
High-sensitivity c-reactive protein (hs-CRP)
The high-sensitivity c-reactive protein test (hs-CRP) is used as a marker for inflammation. Measuring and charting CRP values can help determine disease progress or the effectiveness of treatments. It can be a warning sign that the body is reacting to something in the diet (e.g. gluten or refined carbs) or body (e.g. infection or chronic inflammation).
CRP is synthesized by the liver as part of the body’s immune complement system, binding to dead or dying cells to help mark them for clearance by phagocytes.
When to check
Baseline. Repeat every 3-6 months for patients who have an inflammatory condition or who are doing significant dietary experiments.
Standard reference ranges
<3 mg/L
Increased values
Higher than normal values may be caused by:
- Atherosclerosis
- Rheumatoid arthritis
- Autoimmune disease
- Infections
- Inflammatory disease
- Malignancy
Low-carb considerations
CRP levels often reduce in patients on low carb diets, providing tangible evidence that inflammation is decreasing and reinforcing commitment to the lifestyle.
Nuclear magnetic resonance (NMR) lipoprofile
A nuclear magnetic resonance (NMR) lipoprofile, or particle test, is a more detailed test of circulating cholesterol that can help refine a patient’s risk of developing cardiovascular disease. This test can be particularly helpful in measuring low-density lipoproteins (LDL) particles, and evaluating them according to their number and particle size.
When to check
Baseline and annually, or sooner if significant dietary changes are being made.
Low-carb considerations
This test is not available everywhere and may not be covered by insurance. Low-carb diets tend to be effective at reducing the amount of small LDL and increasing the overall LDL size. For those who see a rise in their LDL, the so-called hyper-responders, this test may be helpful to ensure there is a rise in the larger, less dense particles and not in the smaller, more concerning particles.
Thyroid stimulating hormone (TSH)
A thyroid-stimulating hormone test checks whether a patient’s thyroid may be underactive or overactive. The test measures blood levels of TSH, secreted by the pituitary to tell the thyroid how much T3 and T4 to produce. An abnormal TSH value does not always indicate hypo or hyperthyroidism. The interpretation of TSH also depends on blood levels of T3 and T4, as well as the clinical symptoms of thyroid conditions and a physical exam.
When to check
Baseline if the patient has a history of hypothyroidism and Hashimoto’s disease, or takes thyroid medication. Repeat every 3 months until levels stabilize, then repeat annually.
Standard reference ranges
Increased values
Higher than normal values may be caused by:
- Hypothyroidism
- Hashimoto’s disease
- The recovery phase of acute illness
Decreased values
Lower than normal values may be caused by:
- Acute medical or surgical illness
- Grave’s disease
- Hyperthyroidism
- Drug treatments, e.g. glucocorticoids or somatostatin analogs
Low-carb considerations
Patients with pre-existing hypothyroidism or Hashimoto’s disease may experience mild or even significant improvement of their thyroid function on a low carb diet.
A frequent symptom of hypothyroidism is fatigue. This can be confused with a sub-optimal intake of calories, protein, or the induction or keto-flu. Checking TSH levels can help determine why your patient may be experiencing these symptoms.
Uric acid
The uric acid test helps assess the patient on the metabolic syndrome spectrum. A product of purine breakdown, found in urine, uric acid levels are an independent risk factor for diabetes. Excess uric acid can cause gout and kidney stones. Levels can be affected by diet, alcohol consumption, and kidney function.
When to check
Baseline. Repeat every 6 months in patients with a history of gout.
Standard reference ranges
Low-carb considerations
Uric acid can temporarily increase when patients start low-carb. This may be observed within the first six weeks as patients transition into a fat-burning state, with eventual normalization.
Patients with a history of gout may be at increased risk of triggering another one in the first six weeks of going low carb, but studies to date do not confirm this. The pros and cons of prophylaxis treatment should be discussed with patients at risk for gout.
Vitamin B12
Vitamin B12 (cobalamin) levels may help distinguish reasons for fatigue or other symptoms that may be related to diet, malabsorption, or malnutrition. Vitamin B12 is a water-soluble vitamin that plays a key role in the functioning of the brain and nervous system and the formation of red blood cells. It is involved in the metabolism of every cell of the human body, especially DNA synthesis and regulation, fatty acid synthesis, and energy production.
When to check
Baseline for vegetarian and vegan patients, and patients with a history of gastric bypass or lap-band surgery. If abnormal, repeat every 6 months.
Standard reference ranges
Decreased values
Lower than normal values may be caused by:
- Bowel resection (gastric bypass surgery, gastrectomy)
- Celiac disease
- Crohn’s disease
- Dietary deficiency (veganism)
- Helminth infection
- Hyperthyroidism
- Malabsorption
- Drug side effects, e.g, metformin, oral contraceptive pill
- Pernicious anemia
- Pregnancy
Low-carb reference considerations
A low carb diet rich in animal products should provide adequate B12 levels. Of note, if a patient has macrocytic anemia related to a B12 deficiency, this can lead to an inaccurate (higher) HbA1c test result. One of the most frequent symptoms of vitamin B12 deficiency is fatigue and mental fog, which can be confused with a sub-optimal intake of protein, calories, or the induction or keto-flu. Checking vitamin B12 levels can help determine why your patient may be experiencing these symptoms.