Clinical Reference Intervals Across Disciplines

The chemistry entries below show how far published intervals can differ between laboratories. The examples come from the Mayo Clinic Laboratories catalog. Age partitions apply to several tests, and adult creatinine and blood urea nitrogen (BUN) intervals are sex-specific in that catalog.^{33 Mayo Clinic Laboratories. Comprehensive metabolic panel, serum; creatinine, serum; blood…}

Fasting plasma glucose and hemoglobin A1c (HbA1c) are commonly compared with clinical decision limits. In nonpregnant people, fasting plasma glucose of 100 to 125 mg/dL (5.6 to 6.9 mmol/L) or HbA1c of 5.7% to 6.4% (39 to 47 mmol/mol) meets the American Diabetes Association's 2026 criteria for prediabetes. Label those limits as decision limits. They are not central 95% reference intervals. An HbA1c of 5.7% converts to about 39 mmol/mol with the master equation of the National Glycohemoglobin Standardization Program (NGSP) and the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and whole-number rounding.^{4,54 American Diabetes Association Professional… 2. 2026.5 National Glycohemoglobin Standardization… IFCC standardization of HbA1c.}

Arterial blood-gas intervals require the specimen source, inspired oxygen, altitude, temperature handling, and analyzer to be known. One sea-level adult teaching set lists the following values. Venous and capillary samples require their own intervals.^{66 MedlinePlus. Blood gases. 2024.}

Calculated arterial bicarbonate and serum or plasma total CO₂ are related measurements from different test systems. Keep their intervals labeled by specimen and method.

Current Mayo catalog examples for hematinic and hemolysis-related tests also show why one broad adult range can be misleading.^{77 Mayo Clinic Laboratories. Haptoglobin, serum; iron and total iron-binding capacity, serum…}

Ferritin and haptoglobin are acute-phase reactants, and inflammation can increase either result without disease in the marrow or blood. Serum folate below 4.0 µg/L suggests possible deficiency and requires clinical correlation. Laboratory guidance prefers serum folate to red-cell folate because red-cell assays vary too much between analyzers. Plasma-free hemoglobin requires plasma. Clotting liberates hemoglobin from red cells and makes serum unsuitable for this interval. The erythrocyte sedimentation rate (ESR) is also method-specific. An Oregon Health & Science University (OHSU) Westergren example sets the upper limit at 15 mm/h for males through age 50 and 20 mm/h for older males, with 20 mm/h for females through age 50 and 30 mm/h for older females.^{88 Oregon Health & Science University. Erythrocyte sedimentation rate.}

CBC intervals depend on the analyzer and the reference population used to derive them. The table shows selected adult Mayo intervals from that catalog's one analyzer system. Relative differential percentages and absolute counts have separately derived intervals, so do not calculate one set of endpoints from the other.^{99 Mayo Clinic Laboratories. Complete blood cell count with differential, blood.}

Mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, mean platelet volume, and reticulocyte results require the interval reported for the specific analyzer or assay. Adult nucleated RBCs are generally expected to be absent from peripheral blood. The Red Cell, Reticulocyte, and Absolute Count Calculations lesson covers the related formulas.

Pediatric partitions are narrower than the broad age bands used in many teaching tables. The same Mayo catalog separates the first 14 days, several intervals during early infancy, childhood, and adolescence. Selected values show the developmental pattern.^{99 Mayo Clinic Laboratories. Complete blood cell count with differential, blood.}

Hemoglobin and MCV start high after birth and fall through early infancy, and hemoglobin rises again toward the later-childhood and adult intervals. WBC, platelet, RDW, reticulocyte, and nucleated-RBC intervals also change after birth. Gestational age and postnatal age both affect neonatal intervals, so compare a pediatric result with the exact age and sex partition printed on its report.

Partitioned-interval example

Suppose a 34-year-old woman has a hemoglobin of 12.0 g/dL and a serum ferritin of 9 µg/L. The laboratory information system compares each result with one partition, the age and sex subgroup that has its own interval, selected from her registered age and sex. A result outside its interval is flagged. Her results use the adult female and female 18 to 50 years rows of the table.^{7,97 Mayo Clinic Laboratories. Haptoglobin, serum; iron and total iron-binding capacity, serum…9 Mayo Clinic Laboratories. Complete blood cell count with differential, blood.}

In this example, applying one of the other listed partitions can produce a low flag without changing the measurement. A different laboratory method or interval can also change the flag. When a flag disagrees with the rest of the CBC or with prior results, confirm that the registered age and sex match the specimen and the order before you release the result. The interval printed beside a result shows which comparison was made.

Neither unflagged result excludes disease. A reference interval describes the central 95% of its reference population. Whether 9 µg/L indicates depleted iron stores depends on a decision limit, a cutoff chosen from clinical evidence. The Iron deficiency section explains that cutoff. Ferritin is also an acute-phase reactant, so a result inside the interval during inflammation does not exclude depleted stores. Review the CBC in the patient's age and physiologic setting, as described under Establishing the anemia pattern, before interpreting any single flag.

Hemoglobin fractions change rapidly during infancy. The capillary-electrophoresis and HPLC example below comes from one current Mayo assay. Transfusion history, gestational age, postnatal age, and a hemoglobin variant can alter the pattern.^{1010 Mayo Clinic Laboratories. Hemoglobin electrophoresis evaluation, blood.}

Bone-marrow differentials depend on aspirate quality, dilution with peripheral blood, cell-category definitions, the number of cells counted, and observer classification. A study of 236 healthy adult donors used first aspirates, with three investigators counting 200 cells each for a 600-cell total, to derive the following 95% reference intervals.^{1111 Parmentier S, et al. Reevaluation of reference values for bone marrow differential… 2020.}

The study classified one to three megakaryocytes per low-power field as its expected finding. Laboratories may use qualitative or semiquantitative megakaryocyte categories, so apply this count only with the study method.

Coagulation intervals change during infancy and childhood. Vitamin K-dependent factors II, VII, IX, and X, antithrombin, protein C, and protein S are lower in early life in many assay systems. Factor VIII and von Willebrand factor do not follow that pattern. Prothrombin time may be similar to the adult interval, and activated partial thromboplastin time can be longer in young children. Reagent and analyzer effects can be as large as age effects, so each laboratory needs age-appropriate intervals for its own system.^{1212 Toulon P. Developmental hemostasis. 2016.}

The coagulation intervals below come from one Mayo assay system and apply to that system. They cannot be used as a composite panel for another system.^{1313 Mayo Clinic Laboratories. Factor VIII activity, plasma; factor IX activity, plasma…}

Factor activity reported as a percentage and as U/dL has the same numerical value only when the calibration assigns 100% activity to 100 U/dL. D-dimer also requires its assay and unit convention, including fibrinogen-equivalent units or D-dimer units. Activated protein C resistance, antiphospholipid antibody, factor XIII, prekallikrein, high-molecular-weight kininogen, plasminogen-activator inhibitor 1, and homocysteine cutoffs are method- or decision-specific. The report and current interpretive procedure supply the applicable limits.

CSF intervals depend on age, collection site, concurrent blood results, and method. In a study of febrile infants without bacterial meningitis, the 95th-percentile lumbar CSF WBC counts were 19 cells/µL at age 0 to 28 days and 9 cells/µL at age 29 to 56 days. These study limits apply to that study population only.^{1414 Kestenbaum LA, et al. Defining cerebrospinal fluid white blood cell count reference… 2010.}

Current Mayo and Children's Minnesota laboratory references provide the following examples for older patients. CSF glucose is interpreted against a blood glucose collected at about the same time, and is commonly about 60% of the blood value.^{15,1615 Mayo Clinic Laboratories. Glucose, spinal fluid; total protein, spinal fluid; IgG and…16 Children's Minnesota. CSF count and differential.}

Differential percentages are unstable when only a few cells are present. Mononuclear cells usually predominate in older children and adults. Specimens from neonates and young infants can contain more neutrophils. An increased CSF eosinophil count prompts investigation for parasitic and fungal infection and for noninfectious causes, including a shunt reaction, medication, and malignancy. Interpret it with the absolute count, collection site, and clinical setting.^{1717 Lo Re V 3rd, et al. Eosinophilic meningitis. 2003.} The Body-Fluid Cell Counts and Cerebrospinal Fluid lesson covers specimen handling and detailed interpretation.

Light's criteria classify a pleural effusion as an exudate when at least one of these conditions is met:^{1818 Light RW, et al. Pleural effusions. 1972.}

• pleural-fluid total protein ÷ serum total protein >0.5
• pleural-fluid lactate dehydrogenase (LDH) ÷ serum LDH >0.6
• pleural-fluid LDH > two-thirds of the serum LDH upper reference limit

Use serum and pleural-fluid results collected close together. Keep full precision while comparing a ratio with its cutoff, then round the displayed result. For example, pleural-fluid protein of 3.4 g/dL and serum protein of 6.0 g/dL give:

protein ratio = 3.4 ÷ 6.0 = 0.5666..., displayed as 0.57

The unrounded ratio exceeds 0.5, so the protein criterion is met. Light's criteria apply to pleural fluid. Pericardial and other serous fluids require their own clinical and laboratory criteria.

The serum-ascites albumin gradient (SAAG) supports the etiologic classification of ascites:

SAAG (g/dL) = serum albumin (g/dL) − ascitic-fluid albumin (g/dL)

Collect the serum and ascitic-fluid specimens at about the same time. A SAAG of at least 1.1 g/dL supports portal hypertension in the appropriate clinical setting. A lower value suggests another mechanism.^{1919 Biggins SW, et al. Diagnosis, evaluation, and management of ascites, spontaneous… 2021.} If serum albumin is 3.2 g/dL and ascitic-fluid albumin is 1.7 g/dL:

SAAG = 3.2 − 1.7 = 1.5 g/dL

The result meets the 1.1 g/dL cutoff used to classify ascites. This cutoff is a decision limit. It was not derived as a reference interval from a healthy population. Pleural-fluid triglyceride above 110 mg/dL (1.24 mmol/L) supports a chylous effusion, and demonstration of chylomicrons establishes its chylous nature. Pleural-fluid cholesterol above 200 mg/dL (5.2 mmol/L), absent chylomicrons, and cholesterol crystals support a pseudochylous effusion. Thresholds for peritoneal fluid differ, and appearance alone cannot make the distinction.^{20,2120 Roberts ME, et al. British Thoracic Society guideline for pleural disease. 2023.21 Mayo Clinic Laboratories. Chylomicron screen, body fluid.}

A clinical-methods reference describes normal joint fluid as clear or pale yellow, high in viscosity, and without crystals.^{2222 Walker HK, et al. Joint fluid. 1990.} Appearance and viscosity are qualitative observations. Aspirated volume and RBC count depend strongly on collection, and the traditional string test is a subjective viscosity estimate. The cell-count values below come from one current Mayo body-fluid method.^{2323 Mayo Clinic Laboratories. Cell count and differential, body fluid.}

Mononuclear cells predominate in a low-cellularity sample. A traumatic collection can add RBCs and peripheral leukocytes. Protein, hyaluronate, glucose, uric acid, and lactate results are method-specific. When synovial glucose is measured, compare it with a serum or plasma glucose collected near the time of the fluid sample.

Urine concentration varies widely with hydration and renal concentrating ability. Current Mayo refractometry and freezing-point-osmometry examples include specific gravity of 1.002 to 1.030, urine pH of 4.5 to 8.0, osmolality of 50 to 750 mOsm/kg for infants younger than 12 months, and 150 to 1,150 mOsm/kg for patients age 12 months and older.^{2424 Mayo Clinic Laboratories. Specific gravity, urine; osmolality, urine; pH, urine.} Protein, glucose, ketones, bilirubin, nitrite, and leukocyte esterase are expected to be negative on a routine reagent strip, subject to the strip's detection limits and interferences. Urobilinogen is usually low or trace. Microscopy intervals depend on centrifugation, volume, field size, imaging system, and reporting convention. A few hyaline casts can occur with dehydration or exercise. Granular casts require morphologic and clinical interpretation. The Specific Gravity and Osmolality and Urine Microscopy lessons cover these method effects. The Urine Reagent-Strip Chemistry and Confirmatory Testing lesson covers strip reactions, detection limits, and confirmatory methods.

Interpret 24-hour urine volume and osmolality with intake, fluid losses, collection duration, and collection completeness. The Timed Urine and Renal Clearance Calculations lesson covers the required time and volume arithmetic.

Formed brown stool and absent fecal leukocytes are usual qualitative findings. An immunochemical fecal occult-blood reference result is negative, with method-specific detection limits. For quantitative fecal fat, one current Mayo method requires a 100 to 150 g daily fat intake and a 24- to 96-hour collection. Its adult timed interval is 2 to 7 g fat per 24 hours. The same catalog reports 0% to 19% fat for a random stool method. The timed result is normalized to 24 hours, and the test does not identify the cause of fat malabsorption by itself.^{2525 Mayo Clinic Laboratories. Fat, feces; Occult blood, feces.}