7.3 Interpreting Biochemical and Laboratory Data in Nutrition Assessment
Key Takeaways
WHO's 2024 cutoffs define anemia as hemoglobin below 13.0 g/dL in men, 12.0 g/dL in non-pregnant women, 11.0 g/dL in children aged 24-59 months, and 10.5 g/dL in children aged 6-23 months.
Iron deficiency anemia shows low ferritin, low serum iron, low transferrin saturation, high total iron-binding capacity, and a low MCV; anemia of chronic disease shows low iron but normal or high ferritin.
A high MCV (above about 100 fL) suggests vitamin B12 or folate deficiency; elevated methylmalonic acid points to B12 rather than folate.
Dehydration raises BUN out of proportion to creatinine (ratio above about 20:1) and concentrates hemoglobin, hematocrit, albumin, and sodium; overhydration dilutes them.
Glucose in mg/dL is divided by 18 to convert to mmol/L, and cholesterol in mg/dL is divided by about 38.7.
The table of specifications asks examinees to know simple diagnostic tests and to evaluate and interpret biochemical and laboratory data. Reference ranges differ slightly between laboratories, so use the range given in the question; the values below are typical adult ranges.
Complete Blood Count and Anemia
| Test | Typical adult range | Nutrition significance |
|---|---|---|
| Hemoglobin (Hb) | Men about 13.5-17.5 g/dL; women about 12-15.5 g/dL | Low in anemia; high in dehydration |
| Hematocrit (Hct) | Men about 41-50%; women about 36-44% | Changes with hydration |
| Mean corpuscular volume (MCV) | 80-100 fL | Low (microcytic): iron deficiency, thalassemia trait, B6 deficiency; high (macrocytic): B12 or folate deficiency, alcohol |
| Mean corpuscular hemoglobin concentration (MCHC) | 32-36 g/dL | Low (hypochromic) in iron deficiency |
| White blood cells | 4,500-11,000 cells/mm3 | Total lymphocyte count below 1,500 suggests depleted immunity |
WHO hemoglobin cutoffs for anemia (2024 guideline):
| Group | Anemia if Hb below |
|---|---|
| Children 6-23 months | 10.5 g/dL |
| Children 24-59 months | 11.0 g/dL |
| Children 5-11 years | 11.5 g/dL |
| Children 12-14 years and non-pregnant women 15-65 years | 12.0 g/dL |
| Pregnant women, first and third trimesters | 11.0 g/dL |
| Pregnant women, second trimester | 10.5 g/dL |
| Men 15-65 years | 13.0 g/dL |
The 2024 guideline lowered the cutoffs for children 6-23 months and for the second trimester of pregnancy, which older references list as 11.0 g/dL.
Iron, Vitamin B12, and Folate Studies
| Pattern | Ferritin | Serum iron | TIBC | Transferrin saturation | MCV |
|---|---|---|---|---|---|
| Iron deficiency anemia | Low | Low | High | Low (below about 16%) | Low |
| Anemia of chronic disease (inflammation) | Normal or high | Low | Low or normal | Low or normal | Normal or low |
| Iron overload | High | High | Low | High | Normal |
- Ferritin is the best single marker of iron stores, but it is also an acute-phase protein, so infection or inflammation can raise it and hide iron deficiency. Check C-reactive protein at the same time.
- Vitamin B12 deficiency: macrocytic anemia, low serum B12, high methylmalonic acid and homocysteine.
- Folate deficiency: macrocytic anemia, low folate, high homocysteine, normal methylmalonic acid.
Electrolytes and Acid-Base
| Test | Typical range | High (common causes) | Low (common causes) |
|---|---|---|---|
| Sodium | 135-145 mEq/L | Dehydration, water loss | Fluid overload, SIADH, diuretics |
| Potassium | 3.5-5.0 mEq/L | Kidney failure, ACE inhibitors, tissue breakdown | Diuretics, vomiting, diarrhea, refeeding |
| Chloride | 98-106 mEq/L | Dehydration, saline infusion | Vomiting |
| Bicarbonate | 22-28 mEq/L | Metabolic alkalosis, vomiting | Metabolic acidosis, diarrhea, kidney failure |
Kidney Function Tests
| Test | Typical range | Interpretation |
|---|---|---|
| Blood urea nitrogen (BUN) | 7-20 mg/dL | Rises with high protein intake, GI bleeding, catabolism, dehydration, and kidney failure; falls with low protein intake and liver failure |
| Serum creatinine | Men about 0.7-1.3 mg/dL; women about 0.6-1.1 mg/dL | Reflects muscle mass and glomerular filtration |
| BUN:creatinine ratio | About 10-20:1 | Above 20:1 suggests dehydration or prerenal causes |
| Estimated GFR | 90 mL/min/1.73 m2 or more is normal | Used to stage chronic kidney disease |
| 24-hour urine creatinine | Reflects muscle mass | Used in the creatinine-height index |
Liver Function Tests
| Test | What it shows |
|---|---|
| ALT and AST | Liver cell injury; AST:ALT above 2:1 suggests alcoholic liver disease |
| Alkaline phosphatase and GGT | Cholestasis (bile flow obstruction); alkaline phosphatase is also high in bone growth and bone disease |
| Bilirubin | Jaundice from hemolysis, liver disease, or bile duct obstruction |
| Albumin and prothrombin time (INR) | Synthetic function; a prolonged prothrombin time may reflect vitamin K deficiency or liver failure |
| Ammonia | Raised in hepatic encephalopathy |
Glucose and Lipids
| Test | Normal | Prediabetes or borderline | Diabetes or high |
|---|---|---|---|
| Fasting plasma glucose | Below 100 mg/dL | 100-125 mg/dL | 126 mg/dL or more |
| HbA1c | Below 5.7% | 5.7-6.4% | 6.5% or more |
| Total cholesterol | Below 200 mg/dL | 200-239 mg/dL | 240 mg/dL or more |
| Triglycerides | Below 150 mg/dL | 150-199 mg/dL | 200 mg/dL or more |
| HDL cholesterol | 60 mg/dL or more is protective | Below 40 mg/dL (men) or 50 mg/dL (women) is low |
HbA1c reflects average glucose over about 2-3 months, the lifespan of red blood cells.
Minerals, Uric Acid, and Urinalysis
| Test | Typical range | Notes |
|---|---|---|
| Total calcium | 8.5-10.5 mg/dL | Correct for low albumin; ionized calcium is the active form |
| Phosphorus | 2.5-4.5 mg/dL | High in kidney failure; very low in refeeding syndrome |
| Magnesium | 1.7-2.2 mg/dL | Low with diuretics, alcohol use, diarrhea, refeeding |
| Uric acid | Men about 3.4-7.0 mg/dL; women about 2.4-6.0 mg/dL | High in gout, kidney failure, high purine or alcohol intake |
| Urine ketones | Negative | Positive in starvation, very-low-carbohydrate diets, diabetic ketoacidosis |
| Urine glucose | Negative | Positive when blood glucose exceeds the renal threshold (about 180 mg/dL) |
| Urine protein (albumin) | Negative | Albuminuria signals kidney damage in diabetes and hypertension |
Population Biomarkers
Nutrition surveys use biomarkers to judge micronutrient status in populations:
- Hemoglobin for anemia prevalence.
- Serum retinol below 20 µg/dL (0.70 µmol/L) for vitamin A deficiency.
- Median urinary iodine concentration for iodine status; WHO considers 100-199 µg/L adequate in school-age children.
- Serum ferritin for iron stores, adjusted for inflammation.
Factors That Confound Laboratory Values
- Hydration: dehydration concentrates values; overhydration and edema dilute them.
- Inflammation: lowers albumin, prealbumin, transferrin, and serum iron; raises ferritin and C-reactive protein.
- Medications: diuretics change potassium and magnesium; steroids raise glucose.
- Kidney and liver disease: change protein, urea, and creatinine levels independently of intake.
Useful Unit Conversions
| Measure | Conventional to SI |
|---|---|
| Glucose | mg/dL divided by 18 = mmol/L |
| Cholesterol | mg/dL divided by 38.7 = mmol/L |
| Triglycerides | mg/dL divided by 88.6 = mmol/L |
| Creatinine | mg/dL times 88.4 = µmol/L |
Example: fasting glucose of 126 mg/dL / 18 = 7.0 mmol/L, the diabetes threshold.
A 24-year-old woman has hemoglobin 10.2 g/dL, MCV 72 fL, serum ferritin 8 ng/mL, and a high total iron-binding capacity. What do these results indicate?
Iron deficiency anemia
Vitamin B12 deficiency
Anemia of chronic inflammation
Folate deficiency
An elderly patient has BUN 42 mg/dL, creatinine 1.4 mg/dL, sodium 149 mEq/L, and a high hematocrit, with dry mucous membranes. What is the most likely cause of these values?
Overhydration with dilution of plasma
Dehydration with prerenal azotemia
Severe protein restriction
Liver failure with low urea synthesis
A patient's fasting plasma glucose is 162 mg/dL. What is this value in mmol/L?
About 4.2 mmol/L
About 5.6 mmol/L
About 2.9 mmol/L
About 9.0 mmol/L
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