Clinical Chemistry: Electrolytes, Acid-Base Balance, and Glucose Testing

Practice flashcards covering the anion-cation gap, diabetic ketoacidosis, quality control rules, blood gas buffering mechanisms, and glucose specimen collection protocols.

What three physiological parameters does the human body prioritize controlling in order of hierarchy?

The body prioritizes controlling pressure first, pH second, and charge third.

Which primary cations and anions are present in blood electrolyte balance?

The primary cations are sodium, potassium, calcium, and magnesium; the primary anions are chloride, bicarbonate ($HCO_3$), proteins, and organic acids.

What is the formula used to calculate the anion gap when potassium is included?

$$\text{Anion Gap} = ([Na^+] + [K^+]) - ([Cl^-] + [HCO_3^-])$$

What is the normal reference range for the anion gap when potassium is included in the equation?

$$10\text{ to }20\,mmol/L$$

Why are magnesium and calcium excluded from standard anion gap calculations?

Their reference ranges are very small (e.g., magnesium is $0.7\text{ to }1.0\,mmol/L$) compared to sodium ($135\text{ to }145\,mmol/L$), making their mathematical contribution negligible.

Why is potassium frequently removed from the anion gap equation in clinical practice?

Potassium has a narrow, tightly regulated range ($3.5\text{ to }5.0\,mmol/L$), remains relatively constant, and is highly prone to false elevation from pre-analytical hemolysis.

What is the reference range for the anion gap when potassium is excluded from the calculation?

$$7\text{ to }16\,mmol/L$$

How does the accumulation of organic acids, such as ketones, affect bicarbonate and the anion gap?

Organic acids increase, which displaces and decreases bicarbonate ($HCO_3$) to preserve electrical neutrality, causing the calculated anion gap to increase.

How do medical laboratory technologists use patient anion gaps as an internal quality control check?

They track patient gap trends; if an entire run of routine outpatients displays elevated gaps, it indicates an analytical error in testing sodium, potassium, chloride, or bicarbonate.

How is an analytical run defined in the clinical chemistry laboratory for quality control purposes?

An analytical run is defined as a period of no less than $24\text{ hours}$, requiring a minimum of two levels of QC to be run.

What is the relationship between blood pH and metabolic bicarbonate ($HCO_3$)?

They are directly proportional; when bicarbonate produced or regulated by the kidneys drops, blood pH decreases accordingly.

What is the relationship between blood pH and respiratory $pCO_2$?

They are inversely (indirectly) proportional; when $pCO_2$ regulated by the lungs increases, blood pH decreases.

What is the normal physiological reference range for human blood pH?

$$7.35\text{ to }7.45$$

What is Kussmaul breathing, and why does it occur in diabetic ketoacidosis?

It is an involuntary, rapid gasping respiratory pattern that compensates for metabolic acidosis by blowing off $pCO_2$ through the lungs to raise blood pH.

Why do patients in diabetic ketoacidosis develop high serum potassium levels?

Excess hydrogen ions ($H^+$) are buffered inside red blood cells by hemoglobin; to balance the excess positive charge, red cells extrude potassium into the serum.

What is the ideal timeframe for centrifuging a blood specimen for glucose testing, and what is the maximum acceptable limit?

The ideal time is within $30\text{ minutes}$, with an acceptable upper limit of $60\text{ minutes}$ before in vitro glycolysis distorts the result.

What are the common collection tube types and additives used for blood glucose analysis?

Plasma separator tubes (PST with lithium heparin), plain lithium heparin tubes, serum separator tubes (SST), plain red top tubes, EDTA tubes, and gray top tubes with sodium fluoride and potassium oxalate.

How do gross hemolysis and extremely high white blood cell counts affect measured blood glucose levels?

Gross hemolysis falsely decreases glucose as a negative interferent; high white blood cell counts (e.g., $300 \times 10^9/L$) rapidly consume glucose in vitro via active metabolism.

How much does capillary blood glucose differ from venous blood glucose on point-of-care glucometers?

Capillary blood glucose differs by approximately $5\%\text{ to }10\%$ from venous blood glucose.

Why does severe polycythemia or an elevated hemoglobin (e.g., $220\,g/L$) cause erroneous glucometer results?

The high volume of red blood cells crowds the test pad and displaces glucose molecules, interfering with the meter's ability to measure glucose accurately.

What are the standard glucose dosages administered for oral glucose tolerance testing in non-pregnant adults versus pregnant patients?

Non-pregnant adults receive a $75\,g$ glucose drink, whereas pregnant patients undergoing gestational screening receive a $50\,g$ drink.

What is a two-hour post challenge (2-hour PC) glucose test?

A test where the patient eats a balanced meal in the cafeteria and has their blood glucose measured exactly $2\text{ hours}$ later to assess glucose handling.

What are the established reference ranges for fasting glucose and the diagnostic threshold for random glucose?

Normal fasting glucose is $4\text{ to }6\,mmol/L$, while random glucose is diagnostic when elevated above $11\,mmol/L$.

What instructions must be given to a patient preparing for a fasting blood glucose test?

The patient must fast for $8\text{ to }12\text{ hours}$ (a minimum solid $8\text{ hours}$), consuming only small sips of plain water and avoiding all food, candy, gum, coffee, tea, and juice.