Clinical Chemistry

General Calculations

Standard Deviation (SD) = ∈(x−x)2n−1\sqrt{\frac{\in\left(x-x\right)^2}{n-1}}

  • 1 SD = 68% of values; excludes 32%

  • 2 SD = 95% of values; excludes 5%

  • 3 SD = 99.7% of values; excludes 0.3%

Coefficient of Variation (CV) = SD⋅100mean⁡\frac{SD\cdot100}{\operatorname{mean}}

  • expressed as a percentage of the mean

  • lower CV = more precise method

Beer’s Law: Cu=AuAs⋅CsCu=\frac{Au}{As}\cdot Cs

mg/dL to mEq/L = mgdl(atomicvalence)⋅10\frac{mgdl}{\left(\frac{atomic}{valence}\right)}\cdot10

  • equivalent weight = atomicvalence\frac{atomic}{valence}

mg/dL to mmol/L = mgdlMW⋅10\frac{mgdl}{MW}\cdot10

Celsius to Fahrenheit: F = 95⋅C+32\frac95\cdot C+32

Fahrenheit to Celsius: C = (F−32)⋅59\left(F-32\right)\cdot\frac59

Quality Assurance

Preanalytical - specimen labeling, specimen collection, specimen transportation; specimen processing

  • most common source of error in the lab

Analytical - reagents, instrumentation, test procedure, personnel

Postanalytical - verifying results, entering results into LIS, Delta check

Diagnostic sensitivity - measures true positives

Diagnostic specificity - measures true negatives

Instrumentation

Spectrophotometry - measures the wavelength of transmitted light from a solution

  • Beer’s Law: absorbance directly proportional to concentration

Atomic Absorption - light absorbed by ground state atoms

Fluorometry - light emitted from compounds that absorb electromagnetic radiation, become excited, and return to energy states slightly lower than their original energy states

  • wavelength of emitted energy is longer than wavelength of absorbed energy

Turbidimetry - light blocked by particulate matter as light passes through the cuvette by a spectrophotometer

Nephelometry - measures scattered light to measure number and size of particles

Osmometry - total number of dissolve particles in a solution based on colligative properties (freezing point depression)

  • more concentrated solution has lower freezing point

Chromatography - separation of mixtures based on specific differences in physical characteristics

Electrophoresis - separation based on ionic strength; greater negatively charged anions will move furthest towards positively charged pole (anode)

  • electrophoresis can be used for hemoglobin, protein, lipids, isoenzymes

Potentiometry - measure voltage differences between reference and indicator electrodes

  • pH electrode uses hydrogen sensitive glass

  • potassium electrode uses valinomycin

  • reference electrode uses Ag/AgCl

Coulometry - measures the time it takes to titrate out all of the chloride in a specimen

  • reference method for serum chloride

Manometric - measures change in pressure

  • reference method for serum CO2

Carbohydrates

Random glucose: 70-130 mg/dL

  • > 200 md/dL on 2 separate occasions = diabetes

Fasting blood glucose: 70-110 mg/dL

  • > 126 mg/dL on 2 separate occasion = diabetes

2 hr post prandial glucose: 70-110 mg/dL

  • > 200 mg/dL on 2 occasions = diabetes

Urine glucose: negative

  • positive urine glucose = diabetes

  • renal threshold is 160-180 mg/dL

Diabetes mellitus - absolute or relative deficiency of insulin that causes hyperglycemia

  • Type I - absolute insulin deficiency

    • genetically predisposed, acute onset, prone to ketoacidosis, seen in ages under 25, treated with insulin injections  

  • Type II - relative insulin deficiency; insulin resistance

    • associated with obesity, most common type, treated with diet changes and weight loss

Carbohydrate metabolism - carbohydrates broken down into glucose by the enzyme amylase

  • Glucose is stored in the liver as glycogen

  • Excess amounts of glucose are converted into fat and stored as triglycerides in adipose tissue