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ERRATUM FOR THIS: For the action of insulin, please change GLYCOGENOLYSIS to GLYCOGENESIS. Insulin lowers glucose and so it should increase conversion of glucose to glycogen (GLYCOGENESIS).

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Posted in Lecture Notes

Clinical Chemistry – Carbohydrates

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WHAT’S IN HERE?

  • Carbohydrates
  • Glucose and Its Metabolism
  • Hyperglycemia
  • Hypoglycemia
  • Genetic Defects in Carbohydrate Metabolism
  • Laboratory Analysis of Glucose
  • References

DEFINITION

  • Compounds containing C, H and O with general formula Cx(H2O)y
  • Contain C=O and –OH functional groups
  • Derivatives can be formed by addition of other chemical groups such as phosphates, sulfates and amines
  • Commonly called “SUGARS” and use the suffix –ose

CLASSIFICATION

  • Based on four different properties
  • SIZE OF THE BASE CARBON CHAIN
    • TRIOSES: with three (3) carbons
    • TETROSES: with four (4) carbons
    • PENTOSES: with five (5) carbons
    • HEXOSES: with six (6) carbons
  • LOCATION OF THE CO FUNCTION GROUP
    • ALDOSE: has a terminal carbonyl group (O=CH) called an aldehyde group
    • KETOSE: has carbonyl group (O=CH) in the middle linked to two other carbon atoms called a ketone group
  • STEREOCHEMISTRY OF THE COMPOUND
    • STEREOISOMERS: have the same order and types of bonds but different spatial arrangements and different properties
    • ENANTIOMERS: images that cannot be overlapped and are non-superimposable
      • L-isomer: if the configuration of the highest-numbered asymmetric carbon is on the LEFT or if hydroxyl group farthest from the carbonyl carbon is on the LEFT
      • D-isomer: if the configuration of the highest-numbered asymmetric carbon is on the RIGHT or if hydroxyl group farthest from the carbonyl carbon is on the RIGHT
  • NUMBER OF SUGAR UNITS
    • MONOSACCHARIDES
      • Simple sugars that cannot be hydrolyzed to simpler form
      • Examples: glucose, fructose, galactose
    • DISACCHARIDES
      • Formed by two monosaccharides joined by glycosidic linkage
      • Hydrolyzed by disaccharide enzymes (i.e., lactase) produced by the microvilli of the intestine
      • Examples:
        • Maltose = 2 β-D-glucose in 1→4 linkage
        • Lactose = glucose + galactose
        • Sucrose = glucose + fructose
      • OLIGOSACCHARIDES
        • Chaining of 2 to 10 sugar units
      • POLYSACCHARIDES
        • Linkage of many monosaccharide units
        • Yield more than 10 monosaccharides upon hydrolysis
        • Examples: starch, glycogen

 MODELS USED TO REPRESENT CARBOHYDRATES

  • FISCHER: linear formula where the aldehyde or ketone is at the top of the drawing and can be depicted in the D- or L- form
  • HAWORTH: cyclic form that is more representative of the actual structure and is formed when the carbonyl group reacts with an alcohol group on the same sugar to form a ring and can be depicted in the α or β form

CHEMICAL PROPERTIES

  • REDUCING SUBSTANCES
    • Contain a ketone or aldehyde group
    • WITH FREE ANOMERIC CARBON
    • Can reduce other compounds
    • Examples: glucose, maltose, fructose, lactose, galactose
  • NON-REDUCING SUBSTANCES
    • Do not have an active ketone or aldehyde group
    • NO FREE ANOMERIC CARBON
    • Will not reduce other compounds
    • Example: sucrose (table sugar)
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Glucose and Its Metabolism

  • End product of carbohydrate digestion in the intestine
    • Enzymes involved
      • AMYLASE (salivary & pancreatic) – digests nonabsorbable polymers to dextrins and disaccharides
      • MALTASE (from the intestine) – digests disaccharides to monosaccharides
      • SUCRASE & LACTASE – hydrolyze sucrose & lactose respectively
    • FUNCTIONS:
      • Provides energy for life processes
      • The only CHO that can be directly used for energy or stored as glycogen
    • FORMS: ~35% alpha & 65% beta
    • MAJOR METABOLIC PATHWAYS
      • EMBDEN-MEYERHOFF PATHWAY or GLYCOLYSIS
        • Substrate: D-glucose
        • End-products: 2 moles of PYRUVIC ACID, 2 moles NADH and 2 moles of ATP
        • Can occur aerobically or anaerobically
          • If aerobic, pyruvate is formed
          • If anaerobic, lactate is formed
        • Other substrates can enter this pathway at various points
          • Glycerol (from TAG) enters at 3-phosphoglycerate
          • Fatty acids, ketones and some amino acids are converted to acetyl-CoA
          • Other amino acids enter as pyruvates or as deaminated α-ketoacids and α-oxoacids
        • HEXOSE MONOPHOSPHATE SHUNT OR AEROBIC/OXIDATIVE PATHWAY
          • G6P is converted to 6-phosphogluconic acid which permits the formation of NADPH (important to red cells because they lack mitochondria thus incapable of TCA cycle)
          • End-products: pentose phosphate, CO2 and NADPH
        • GLYCOGENESIS
          • Stores glucose as glycogen
          • Converts G6P to G1P
          • G1P → uridine diphosphoglucose→ glycogen by glycogen synthase
          • GLYCOGENOLYSIS – conversion of glycogen to G6P
PATHWAYS IN GLUCOSE METABOLISM
GlycolysisMetabolism of glucose molecule to pyruvate or lactate for production of energy
GluconeogenesisFormation of G6P from noncarbohydrate sources
GlycogenolysisBreakdown of glycogen to glucose for use as energy
GlycogenesisConversion of glucose to glycogen for storage
LipolysisDecomposition of fats
LipogenesisConversion of carbohydrates to fatty acids
  • MAJOR HORMONES CONTROLLING BLOOD GLUCOSE
    • PANCREATIC HORMONES
      • INSULIN – primary hormone for DECREASING blood glucose levels
        • Responsible for the entry of glucose into the cells by enhancing membrane permeability to cells in the liver, muscle and adipose tissues
        • synthesized by β-cells of the pancreas
          • released when glucose levels are high/increased
          • not released when glucose levels are low/decreased
        • EFFECTS:
          • increases glycogenesis, lipogenesis, and glycolysis
          • inhibits glycogenolysis
        • INSULIN IS THE ONLY HORMONE THAT DECREASES GLUCOSE LEVELS and can be referred to as a hypoglycemic agent
      • GLUCAGON – primary hormone for INCREASING blood glucose levels
        • released in response to stress and fasting states
        • synthesized by α-cells of the pancreas
          • released when glucose levels are low/decreased
          • not released when glucose levels are high/increased
        • EFFECTS:
          • increase glycogenolysis and gluconeogenesis
        • can be referred to as a hyperglycemic agent
      • SOMATOSTATIN
        • produced by δ cells of the pancreas
        • EFFECTS: inhibition of insulin, glucagon, growth hormone, and other endocrine hormones.
      • ADRENAL HORMONES
        • CORTISOL
          • produced by the adrenal cortex on stimulation by ACTH
          • EFFECTS: decreases intestinal entry into the cell and increases gluconeogenesis, liver glycogen and lipolysis
        • EPINEPHRINE
          • produced by the adrenal medulla
          • EFFECTS: inhibits insulin secretion, increase glycogenolysis and lipolysis
          • Released during times of stress
        • ANTERIOR PITUITARY HORMONES
          • GROWTH HORMONE
            • EFFECTS: decreases the entry of glucose into the cells
          • ACTH
            • EFFECTS: stimulates the adrenal cortex to release cortisol, increases glycogenolysis and gluconeogenesis
          • THYROID HORMONES
            • T3 & T4
              • EFFECTS: increases glycogenolysis, gluconeogenesis and intestinal absorption of glucose
HORMONAL ACTIVITY AFFECTING SERUM GLUCOSE LEVELS
HORMONESOURCEEFFECTACTION
Insulinβ cells of pancreasstimulates glucose uptake by cells
Glucagonα cells of pancreasglycogenolysis
ACTHAnterior pituitaryinsulin antagonist, glycogenolysis & gluconeogenesis
Growth HormoneAnterior pituitaryinsulin antagonist & glycolysis
CortisolAdrenal cortexinsulin antagonist, gluconeogenesis & lipolysis
HPLPlacentainsulin antagonist
EpinephrineAdrenal medullainhibits insulin secretion, glycogenolysis & lipolysis
T3 & T4Thyroid glandglycogenolysis, gluconeogenesis & intestinal absorption of glucose
Somatostatinδ cells of pancreasinhibits insulin, glucagon, GH

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Hyperglycemia

  • Increase in plasma glucose levels caused by imbalance of hormones
  • DIABETES MELLITUS
    • Group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action or both
    • Categories of Diabetes (According to the ADA/WHO guidelines)
      • Type 1 Diabetes
      • Type 2 Diabetes
      • Other specific types of diabetes
      • Gestational Diabetes Mellitus (GDM)
  • PRIMARY DIABETES MELLITUS
Points of DifferenceTYPE 1TYPE 2
Former namesInsulin Dependent Diabetes Mellitus (IDDM)

Juvenile Onset DM

Brittle DM

Ketosis-prone DM

Non-Insulin Dependent Diabetes (NIDDM)

Maturity Onset DM

Stable DM

Ketosis-resistant DM

Receptor Deficient DM

OnsetBefore 20 y/oOver 40 y/o
Measurable circulating insulinNONELow
Insulin receptorNormal↓ or ineffective
Beta cell massMarkedly ↓Moderately ↓
C-peptide levelsUndetectableDetectable
Incidence10-15%85% (common)
Ketoacidosis*CommonRare
Physique/Stature**Normal or thinOften overweight
Pathogenesis -β-cell destruction

-Absolute insulin deficiency

-Autoantibodies

-Insulin resistance with insulin secretory defect

-Relative insulin deficiency

TreatmentParenteral insulin administraionOral hypoglycemic agent
  • SECONDARY DIABETES MELLITUS – associated with secondary conditions
    • Genetic defects of β-cell function
    • Pancreatic disease
    • Endocrine disease
      • Cushing syndrome – excessive cortisol
      • Pheochromocytoma – epinephrine excess
      • Acromegaly – growth hormone excess
    • Drug or chemical induced
    • Insulin receptor abnormalities
    • Other genetic syndromes
      • Maturity onset diabetes of youth (MODY) – rare; autosomal dominant
  • GESTATIONAL DIABETES MELLITUS (GDM)
    • any degree of glucose intolerance with onset or first recognition during pregnancy
    • due to metabolic or hormonal changes
    • Infants born to mothers with this kind of diabetes are at increased risk to respiratory distress syndrome, hypocalcemia & hyperbilirubinemia

Laboratory Findings in Hyperglycemia

  • INCREASED glucose (plasma & urine), urine specific gravity, serum and urine osmolality
  • Ketonemia and ketonuria
  • DECREASED blood and urine pH (acidosis)
  • Electrolyte imbalance (↓Na+, Cl and ↑K+)
DIAGNOSTIC CRITERIA FOR DIABETES MELLITUS
RPG                    ≥200 mg/dl (11.1 mmol/L) + symptoms of diabetes
Fasting PG         ≥126 mg/dL (7.0 mmol/L)
2-h PG               ≥200 mg/dl (11.1 mmol/L) during OGTT
CATEGORIES OF FASTING PLASMA GLUCOSE
Normal fasting glucose         FPG <110 mg/dL
IMPAIRED fasting glucose     FPG ≥110 mg/dl but <126 mg/dl
Provisional diabetes dx         FPG ≥126 mg/dl
CATEGORIES OF ORAL GLUCOSE TOLERANCE
Normal glucose tolerance   2h PG <140 mg/dL
Impaired gluc. tolerance     2h PG ≥140 mg/dl but <200 mg/dl
Provisional diabetes dx      2h PG ≥200 mg/dl
  • Screening test for GDM
    • Only high-risk patients should be screened for GDM
      • Age older than 25 years
      • Overweight
      • Strong family history of diabetes
      • History of abnormal glucose metabolism
      • History of a poor obstetric outcome
      • Presence of glycosuria
      • Diagnosis of PCOS
      • Member of an ethnic/racial group with a high prevalence of diabetes (e.g. Hispanic American,  Native  American, Asian American, African American, Pacific Islander)
    • METHODS:
      • ONE-STEP APPROACH – immediate performance of a 3h OGTT without prior screening
      • TWO-STEP APPROACH – initial measurement of plasma glucose at 1-hour postload (50g)
        • IF value ≥140 mg/dL (7.8 mmol/L) then do 3-hour OGTT using 100g glucose
        • GDM is diagnosed when any two of the following values are met or exceeded:
          • Fasting: >95 mg/dl
          • 1 hour: ≥180 mg/dl
          • 2 hours: ≥155 mg/dl
          • 3 hours: ≥140 mg/dl

Hypoglycemia

  • Decrease in plasma glucose levels
    • 65-70 mg/dl (3.6-3.9 mmol/L) – plasma glucose concentration at which glucagon and other glycemic factors are released
    • 50-55 mg/dl (2.8-3.0 mmol/L) – symptoms of hypoglycemia appear
    • Warning S/S are all related to CNS
  • Types of Hypoglycemia (Old)
    • Post-absorptive (Fasting) – MORE SERIOUS
      • Islet cell insulinoma
      • Insulin-producing tumors
      • Ethanol induced
      • Propanolol & salicylate
  • Post-prandial (Reactive) – MILD FORM
    • there is spontaneous recovery of glucose level as a result of insulin level returning to normal
    • Excessive release of insulin
    • Gastro-intestinal surgery
CAUSES OF HYPOGLYCEMIA
Patient Appears Healthy 
No coexisting diseaseDrugs

Insulinoma

Islet hyperplasia or NESIDIOBLASTOSIS

Factitial hypoglycemia from insulin or sulfonylurea

Severe exercise

Ketotic hypoglycemia

Compensated coexistent diseaseDrugs
Patient Appears ILL 
Drugs

Predisposing illness

Hospitalized patient

 
  • Diagnostic criteria for INSULINOMA
    • Change in glucose level of ≥25 mg/dl coincident with an insulin level of ≥6 μU/ml
    • C-peptide levels of ≥0.2 nmol/L
    • Proinsulin levels of ≥5 pmol/L
    • β-hydroxybutyric acid of ≤2.7 mmol/L
  • Diagnostic tests for HYPOGLYCEMIA
    • 72 hour fast which requires the analysis of glucose, insulin, C-peptide and proinsulin at 6-hour intervals
    • POSITIVE RESULT: <45 mg/dl; hypoglycemic symptoms appear after 72 hours had elapsed
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Genetic Defects in Carbohydrate Metabolism

  • Glycogen Storage Diseases – deficiency of a specific enzyme that causes alteration of glycogen metabolism
TypesEnzyme DeficientClinical Features
von Gierke’s dse

Type I

Glucose-6-phosphataseSevere fasting hypoglycemia

Lactic acidosis

Pompe’s dse

Type II

α-1,4-glucosidaseAccumulation of ↑ amount of glycogen on all organs

Presence of abnormally LARGE LYSOSOMES

Forbe’s dse

Type III

Debrancher enzymeHypoglycemia, hepatomegaly, seizures and mental retardation
Andersen’s dse

Type IV

Brancher enzymeProgressive liver enlargement or cirrhosis and muscular weakness by age 2

Absence of storage glycogen

Unbranched AMYLOPECTIN

Other enzyme defects/deficiencies that cause hypoglycemia: glycogen synthase, fructose-1-6,biphosphatase, phosphoenolpyruvate carboxykinase and pyruvate carboxylase.

  • Galactosemia – a cause of failure to thrive syndrome in infants; congenital deficiency of one of three enzymes involved in galactose metabolism, resulting in increased plasma galactose levels
    • Galactose-1-phosphate uridyl transferase – MOST COMMON enzyme deficiency
    • Fructose-1-phosphate aldolase deficiency

Laboratory Analysis of Glucose

  • SPECIMEN COLLECTION AND HANDLING
    • Glucose concentration in whole blood is approximately 15% lower than in plasma or serum.
    • Glucose levels decrease approximately 10 mg/dL (7%) per hour in whole blood.
    • Serum or plasma must be separated within 1 hour (Bishop) to prevent substantial loss of glucose by the cellular fraction, particularly if WBC count is elevated. (within 30 minutes – Henry)
    • Glucose is metabolized at a rate of 7 mg/dl/h at room temperature; and 2 mg/dl/h at 4°C
    • Refrigerated serum or plasma is stable up to 48 hours.
    • Sodium fluoride (2 mg/mL) prevents glycolysis (gray top tube) for up to 48 hours.
    • Glycolysis decrease serum glucose by approximately 5-7% per hour (5-10 mg/dl) in normal, uncentrifuged coagulated blood at room temperature.
    • Fasting blood glucose should be obtained after an approximately 10-hour fast (not >16 hours)
    • Fasting plasma glucose values have a diurnal variation with the mean FBG higher in the morning than in the afternoon.
    • Fasting reference range for serum or plasma is 70-110 mg/dL
    • In the fasting state, arterial (capillary) values are 5 mg/dL higher than the venous concentration.
    • Urine glucose analysis (in 24h urine glucose) may be stabilized by addition of a preservative; should be stored at 4°C during collection because 40% of glucose is lost after 24 hours at room temperature.
    • CSF glucose analysis (if will be delayed) must be centrifuged and stored at 4°C-20°C
    • In normal CSF, values are two-thirds (approximately 60-70%) of plasma level.
    • RENAL THRESHOLD for glucose: 180 mg/dl
  • TYPES OF SPECIMEN FOR GLUCOSE ANALYSIS
    • Fasting Blood Sugar – blood collected after 8-10 hours of fasting (NV: 74-106 mg/dl)
    • Random Blood Sugar – test for INSULIN SHOCK (NV: <200 mg/dl)
    • 2 hour Postprandial Blood Sugar
      • Standard load of glucose: 75 grams
      • Glucose measurement taken 2 hours later
      • (NV : <120 mg/dl)
    • Glucose Tolerance Test – multiple blood and urine glucose test
      • Oral GTT
        • Janney-Isaacson (Single Dose)
        • Exton Rose (Divided Oral dose or Double Dose)
        • Not recommended for routine use
        • Fasting and 2h sample are measured except for pregnant patients
        • Adult load is 75g; children: 1.75 g/kg to 75g
        • Factors that affect tolerance
          • Medications (salicylates, diuretics, anticonvulsants, oral contraceptives and corticosteroids)
          • GI surgery
          • Vomiting
          • Endocrine dysfunction
        • Requirements:
          • Patient should be ambulatory
          • Patient must be in unrestricted diet of 150 grams CHO/day for 3 consecutive days prior to the test
          • Patient must be free from undue stress or severe illness
          • Alcohol intake and smoking are not allowed prior to the test
          • Patient should be fasting at least 10 hours and not more than 16 hours
          • Test should be performed in the morning because of hormonal diurnal effect on glucose
        • IVGTT – blood sample is collected every 10 minutes for 1 hour
          • 5g glucose/kg body weight (given within 3 minutes) administered intravenously
          • fasting is also required
          • NV: 1.4 – 2.0 %
          • Indications of IVGTT
            • Patients who are unable to tolerate large CHO load
            • Patients with altered gastric physiology or GI d/o
            • Patients with malabsorption syndrome
  • Self-Monitoring of Blood Glucose (SMBG)
    • Type 1 DM – should monitor blood glucose 3-4 times per day
    • Type 2 DM – optimal frequency is unknown
  • Glycosylated hemoglobin/Glycated hemoglobin/HbA1C
    • hemoglobin compound formed when glucose reacts with amino group of hemoglobin
    • test for long term diabetic control
    • reflects the average blood glucose level for the previous 2-3 months
    • for every 1% change in HbA1c value there is 35 mg/dl (2 mmol/L) change in the mean
    • in presence of hemoglobinopathies, there will be less time for glucose to
    • binding of glucose to HbA1 is irreversible
    • preferred anticoagulant is EDTA
    • NV: 4.5-8.5%
Methods of HBA1c Measurement
Methods based on STRUCTURAL DIFFERENCES
ImmunoassaysPolyclonal or monoclonal antibodies toward the glycated n-terminal group of the β chain of Hgb 
Affinity chromatographySeparates based on chemical structure using borate to bind glycosylated proteinsNot affected by temperature and other hemoglobins
Methods based on CHARGE DIFFERENCES
Ion-exhange chromatographyPositive-charge resin bedHighly affected by temperature and hemoglobinopathies

HbF – ↑

HbS and C – ↓

ElectrophoresisSeparation is based on differences in chargeHbF values >7% interferes
Isoelectric focusingType of electrophoresis using isoelectric point to separatePre-hb A1c interferes
HPLCForm of ion-exchange chromatographySeparates all forms of glycol Hb (a,b,c)
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  • METHODS FOR ANALYSIS
    • CHEMICAL
      • REDUCTION
        • Cupric Ion Reduction
          • FOLIN-WU – measure of ALL REDUCING SUBSTANCES in the blood
            • Reagent that binds with Cu+: phosphomolybdic acid
            • End product: phosphomolybdenum blue
            • End color: blue
          • NELSON SOMOGYI – MEASURE OF TRUE GLUCOSE
            • Reagent that binds with Cu+: arsenomolybdic acid
            • End product: arsenomolybdenum blue
            • End color: blue
          • NEOCUPROINE
            • Reagent that binds with Cu+: neocuproine
            • End product: cuprous-neocuproine complex
            • End color: yellow/yellow orange
          • Ferric Ion ReductionInverse Colorimetry – reduction of yellow ferricyanide to a colorless ferrocyanide by glucose
            • HAGEDORN JENSEN
    • CONDENSATION
      • Orthotoluidine (DUBOWSKI method)
        • can be also used for urine and CSF without protein precipitation
        • Absorbance: 630 nm
        • Reagent: aromatic amine, glacial acetic acid
        • End color: green
        • Interfering substances: galactose and mannose
    • Polarographic Glucose Oxidase
      • measures oxygen consumption with PO2 electrode (Clark)
      • used to avoid interference made by strong oxidizing agents in GOD
      • Molybdate – catalyzes the oxidation of iodide to iodine by H2O2
      • Catalase – catalyzes oxidation of ethanol by H2O2 forming acetaldehyde and H2O
    • Hexokinase
      • Generally accepted as the REFERENCE METHOD
      • MORE ACCURATE THAN HEXOKINASE
        • coupling reaction using G6PD is highly specific
      • Measured by quantitating reduced NADPH formation
      • NADPH is measured directly at 340 nm or coupled to chromogen and measured in visible range
      • Interfering substances: gross hemolysis & extremely elevated bilirubin (cause ↓ values)
      • May be performed using serum or plasma (heparin, EDTA, fluoride, oxalate & citrate)
      • Excellent for glucose determination in urine, CSF and serous fluids

OTHER IMPORTANT TESTS

  • KETONES
    • Produced by the liver through metabolism of fatty acids to provide ready energy source from stored lipids at times of low carbohydrate availability
    • THREE KETONE BODIES
      • Acetone (2%)
      • Acetoacetic acid (20%)
      • Β-hydroxybutyric acid (78%)
    • Causes of increased ketone levels
      • Diabetes Mellitus
      • Starvation/fasting
      • High-fat diets
      • Prolonged vomiting
      • Glycogen storage diseases
    • KETONEMIA – accumulation of ketones in the blood
      • KETONURIA – accumulation of ketones in the urine
      • MEASUREMENT OF KETONES
        • For patients with Type 1 Diabetes, it is recommended during acute illness, stress, pregnancy, or elevated blood glucose levels above 300 mg/dL or when patients have signs of ketoacidosis
        • SPECIMEN: FRESH SERUM or URINE tightly stoppered and analyzed immediately
        • METHODS FOR ANALYSIS:
          • GERHARDT’S TEST – historical test
            • Used FERRIC CHLORIDE reacted with ACETOACETIC ACID to produce a RED color
    • SODIUM NITROPRUSSIDE – more common method
      • Uses SODIUM NITROPRUSSIDE which reacts with ACETOACETIC ACID in an ALKALINE pH to form a PURPLE COLOR
      • If GLYCERIN is also added, ACETONE will be detected
      • Used in urine reagent strips and Acetest tablets
    • ENZYMATIC – newer method adapted in some automated intstruments
      • Uses β-HYDROXYBUTYRATE DEHYDROGENASE to detect either β-HYDROXYBUTYRIC ACID or ACETOACETIC ACID depending on the pH of the solution
        • pH of 7.0 causes the reaction to proceed to the right (decreasing absorbance)
        • pH of 8.5 to 9.5 causes the reaction to proceed to the left (increasing absorbance)

MICROALBUMINURIA

  • Defined as persistent albuminuria in the range of 30 to 299 mg/24 h or an albumin-creatinine ratio of 30 to 300 g/mg
  • Clinical proteinuria or macroalbuminura is established with an albumin-creatinine ratio of ≥300 mg/24h or ≥300 µg/mg
  • Powerful predictor for future development of diabetic nephropathy
  • Annual assessment of kidney function by the determination of urinary albumin is recommended for diabetic patients
  • METHODS FOR MICROALBUMINURIA SCREENING
    • RANDOM SPOT TEST – preferred method
    • 24-HOUR COLLECTION
    • TIMED 4-HOUR OVERNIGHT COLLECTION
  • A patient is determined to have microalbuminuria when two of three specimens collected within a 3- to 6-month period are abnormal.
  • Factors that may elevate the urinary excretion of albumin include exercise within 24 hours, infection, fever, congestive heart failure, marked hyperglycemia, and marked hypertension

ISLET AUTOANTIBODY AND INSULIN TESTING

  • Not currently recommended for routine screening for diabetes diagnosis but in the future it might identify at-risk, prediabetic patients
TESTS FOR CARBOHYDRATE DISORDERS
DIAGNOSTIC TESTSACTION
Fasting Blood SugarNormal – 70-110 mg/dl

Diabetes – >126 mg/dl

2hr Post Prandial Blood Sugar (PPBS)Normal – <126 mg/dl

Diabetes – >200 mg/dl

Post-Loading GlucoseSimilar to PPBS

*Glucose load is standardized

*Diabetics ≥200 mg/dl

Glucose Tolerance Test (GTT) Standard dose = 75g*Diagnostics of diabetes mellitus

>150 mg/dl after 2 hours

>200 mg/dl after 2 hours

*Perform if FBS and PPBS are normal

Intravenouse Glucose Tolerance Test (IVGTT)*Poor absorption (flat curve with OGTT)

*Patient who cannot tolerate large glucose load (vomiting)

O’Sullivan Test

(for gestational diabetes)

*Standard dose 50g

*Probable gestational diabetes

>150 mg/dl at 1 hour

*Follow up with OGTT

TESTS FOR MONITORINGNOTES
Glycosylated hemoglobin*Assessment of long term control

*Average glucose level over 60 days (1-2 months)

Microalbumin*Detects small amounts of protein in urine of diabetic patients to assess renal damage
C peptide of Insulin

(reflects pancreatic insulin secretion)

Normal 1:1 (insulin:C-peptide)

Diabetes > 1:1

C-Peptide ↓after insulin injection

REFERENCES:

  1. Bishop, Michael L., et.al., Clinical Chemistry Techniques, Principles, Correlations, Sixth Edition
  2. PER Handbook
  3. Theriot, Betty, Clinical Laboratory Science Review: Bottom Line Approach
  4. McPherson, Richard, et.al., Henry’s Clinical Diagnosis and Management by Laboratory Methods, 22e