Labs that every dietitian should know

Laboratory data are among the most frequently used and sometimes misunderstood tools in clinical nutrition care. While registered dietitians do not diagnose disease, they interpret lab results within the context of dietary intake, lifestyle patterns, medical history, and overall clinical presentation. The role of a dietitian includes translating those numbers into meaningful nutrition assessment and evidence-based intervention.  

Reference ranges 

Before we get too far, let’s start with a discussion around reference ranges. When reviewing lab results, it’s important to understand what a reference range actually represents. A reference range is typically based on the values measured in a defined group of individuals considered healthy. Most laboratories establish this range by identifying the central 95% of results in that population. This means that about 5% of healthy individuals will fall outside the reference range even when no disease is present.  

Interestingly enough, reference ranges are not universal. They can vary depending on the lab, the equipment used, the testing method, and the characteristics of the population being studied. Things like age, sex, pregnancy status, and biological variation can all influence values. For this reason, a result that is slightly outside of the reference range does not automatically indicate pathology, nor does a value within the range always confirm optimal health.  

It’s also important to note that some laboratory thresholds are based on risk of adverse outcomes rather than population averages. For example, hemoglobin A1C ranges are established by the American Diabetes Association based on complication risk. Chronic kidney disease (CKD) stages use evidence-based estimated glomerular filtration rate (eGFR) ranges outlined by the Kidney Disease Improving Global Outcomes (KDIGO). In both cases, these values reflect clinical risk vs where most people in the population fall.  

Interpreting lab results in a clinical setting  

Interpretation requires more than comparing a number to a reference range. A structured approach helps prevent misinterpretation and ensures that the data is properly applied to both nutrition assessment and interventions.  

• Look at trends instead of isolated values. If the lab is a baseline value, meaning no previous data exists, then obviously there’s nothing to compare it to. In this case, look at symptoms, risk factors, medications, family history, etc.  
• Assess the clinical context. Be aware that certain acute illnesses and inflammation can falsely elevate or decrease several nutrition-related labs. 
• Understand the purpose of the lab result in the first place. Was the lab ordered for screening, diagnosis, or monitoring? 
• Know when to refer. If the lab results fall outside an expected range or suggest something beyond the scope of a dietitian, be sure to flag the result to the healthcare team. 

What are core laboratory markers every dietitian should understand? 

Glycemic markers 

Glycemic markers include fasting blood sugar and hemoglobin A1C.  Markers of glycemic control are among the most encountered laboratory values in clinical practice. They play a big role in assessing metabolic health, diabetes risk, and response to nutrition interventions.  

What to look for:  

• Fasting Plasma Glucose (FPG) – reflects short-term glucose status after fasting 
• Hemoglobin A1C – reflects average glycemic control over the last 3 months 
• Lab values approaching or exceeding American Diabetes Association diagnostic thresholds should signal closer evaluation and interdisciplinary communication 
• A1C goals are monitored closely in individuals diagnosed with diabetes 
• Labs trending upward, even if they are within the reference range, may be a red flag for metabolic risk 

Clinical considerations for dietitians: 

• Evaluate trends whenever available. Gradual changes over time may be more meaningful than a single, isolated value. 
• Interpret results within clinical context. Sleep, stress, acute illness, and medication can influence blood glucose values. 
• Consider related markers, like lipids, anthropometrics, and diet history, to assess broader metabolic context. 
• Assess carbohydrate and fiber intake, as well as overall timing of meals. Consider alcohol intake and physical activity habits as well. 
• Use follow-up lab data to monitor how effective nutrition interventions are. 

Lipid panel 

A standard lipid panel includes total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglycerides. These labs are used to assess cardiovascular risk and overall metabolic health. Interpretation should focus on patterns rather than single values. 

What to look for: 

• LDL-C – primary marker associated with atherosclerotic cardiovascular risk 
• HDL-C – considered protective related to cardiovascular health 
• Triglycerides – responsive to dietary intake and metabolic status 
• Total cholesterol – most meaningful when looked at in the context of LDL-C and HDL-C 

Clinical considerations for dietitians: 

• Elevated triglycerides with low HDL-C may reflect underlying insulin resistance physiology. 
• Assess dietary fat quality, particularly saturated fat intake, and carbohydrate intake while also evaluating overall dietary patterns. 
• Consider weight status, physical activity, and glycemic markers when assessing overall cardiometabolic risk. 
• Use follow-up lipid panel trends to monitor response to nutrition interventions. 

Iron studies and anemia 

These labs include ferritin, serum iron, total iron-binding capacity (TIBC), and transferrin saturation, as well as hemoglobin and hematocrit. Iron deficiency is one of the most common nutritional deficiencies. Dietitians should look at iron studies when evaluating fatigue, poor oral intake, and gastrointestinal (GI) concerns.  

What to look for:  

• Ferritin – primary marker of iron stores 
• Serum Iron and Transferrin Saturation – reflects circulating iron availability 
• Hemoglobin/Hematocrit – indicate oxygen-carrying capacity 
• Mean Corpuscular Volume (MCV) – helps differentiate types of anemia  
• Any value consistent with iron deficiency should result in prompt evaluation of potential sources of blood loss, malabsorption, or inadequate intake 

Clinical considerations for dietitians: 

• Iron studies should be interpreted as a panel, rather than single values. 
• Note that ferritin can reflect iron stores but is also an acute phase reactant and may be elevated in the presence of inflammation. 
• Consider menstrual history, GI symptoms, and dietary iron bioavailability. 
• Persistent iron deficiency warrants investigation for GI blood loss by medical team. 

Kidney function and electrolytes 

Labs such as creatinine, eGFR, blood urea nitrogen (BUN), potassium, and phosphorus are all indicative of kidney health and function. Assessment of labs associated with kidney function is essential in determining appropriate protein, fluid, and mineral recommendations.  

What to look for:  

• Creatinine – reflects muscle metabolism and is used to estimate kidney function 
• eGFR – is an estimate of filtration capacity and is used in staging CKD 
• BUN – influenced by protein intake, hydration status, and renal function 
• Potassium and phosphorus – serum electrolytes commonly increased in reduced kidney function; low levels can be reflective of poor oral intake, medication side effects, or refeeding syndrome 

Clinical considerations for dietitians: 

• It’s best to interpret creatinine alongside eGFR trends, rather than as a single lab value. 
• Creatinine may be lower in patients with reduced muscle mass and may not always reflect preserved kidney function. 
• Dehydration can elevate BUN and generally concentrate lab values. 

Liver enzymes 

Elevations in labs such as alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are often seen in patients with underlying liver conditions. However, they may also be caused by metabolic dysfunction, medication effects, or alcohol use. 

What to look for:  

• ALT – more liver-specific; often elevated in metabolic dysfunction-associated steatotic liver disease 
• AST – found in the liver, but also in muscle; should be interpreted alongside ALT  

Clinical considerations for dietitians: 

• Mild elevations are often associated with metabolic risk and may improve with weight reduction, improved dietary quality, and physical activity.  
• Liver enzymes should be interpreted within metabolic context including glycemic markers, lipid patterns, and anthropometrics. 
• Consider alcohol intake, medication use, and recent strenuous exercise when looking at elevated values. 

Thyroid function

Labs such as thyroid-stimulating hormone (TSH) and free thyroxine (free T4) are reviewed as indicators of thyroid function. TSH and T4 are often used as screening tools in clinical settings. Symptoms of thyroid dysfunction can overlap with nutrition-related concerns, such as fatigue and weight changes.  

What to look for: 

• TSH– primary screening marker for thyroid function 
• Free T4 – assesses circulating thyroid hormone levels; looked at with TSH 
• Abnormal TSH values are used in the diagnosis of hyper- and hypothyroidism, along with accompanying hormone levels  

Clinical considerations for dietitians: 

• Be aware that mild TSH fluctuations can occur and should be interpreted alongside T4 hormone levels and clinical symptoms.  
• Avoid recommending supplementation for thyroid support without clear evidence of deficiency or medical necessity. 
• Weight changes are rarely explained by mild TSH abnormalities alone.  

Conclusion: A dietitian’s role 

Laboratory data is a useful part of a patient’s clinical picture, but interpretation requires more than memorizing reference ranges. When dietitians are able to recognize trends, understand context, and translate findings into individualized nutrition care, lab values become actionable tools rather than simply isolated numbers.  

Across practice settings, a dietitian’s role is to implement evidence-based interventions and collaborate effectively with the entire healthcare team. When integrated with comprehensive nutrition assessment and clinical presentation, lab results strengthen the dietitian’s ability to improve patient outcomes.  

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Pancholia AK, Kabra NK, Gupta R. Laboratory Evaluation of Lipid Parameters in Clinical Practice. Indian Heart J. 2024;76:S29-S32. https://doi.org/10.1016/j.ihj.2024.02.002  

Thyroid Function Tests. American Thyroid Association website. Accessed February 18, 2026. https://www.thyroid.org/thyroid-function-tests/ 

Doles N, Mon MY, Shaikh A, et al. Interpreting Normal Values and Reference Ranges for Laboratory Tests. J Am Board Fam Med. 2025;38(1):174-179. doi:10.3122/jabfm.2024.240224R1