Lab Values for Nurses: cram pack

Formula sheets, the key method steps for every topic and 8 practice problems, with full solutions at the back.

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1Formulas and methods

Lab values & electrolytes

Method

  1. Compare the value with the laboratory’s own reference range, then with the patient’s baseline and trend.
  2. Check the sample: hemolysis, a delay, or blood drawn near an IV can give false results.
  3. Link the value to its typical signs and to the drugs and conditions that cause it.
  4. Look for interactions: low potassium or magnesium increases digoxin toxicity; dehydration raises lithium.
  5. Act on the patient: assess, hold a drug if the level is unsafe, read back critical values and inform the prescriber.

Arterial blood gas interpretation

Normal ranges
pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L
ROME
Respiratory: PaCO₂ opposite to pH; Metabolic: HCO₃⁻ equal to pH
Winter’s formula
PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2
Metabolic alkalosis
PaCO₂ ≈ 0.7·HCO₃⁻ + 21
Anion gap
AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12
Albumin-corrected gap
AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)
Delta ratio
(AG − 12)/(24 − HCO₃⁻)
P/F ratio
PaO₂/FiO₂
Alveolar gas equation
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8
Serum osmolality
2·Na⁺ + glucose + urea (2·Na⁺ + glucose/18 + BUN/2.8)

Method

  1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
  3. Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
  4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
  5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.

Clinical pharmacokinetics

Rate constant and half-life
k = ln 2/t½ = 0.693/t½
First-order decay
C = C₀·e−kt = C₀·(1/2)t/t½
Two levels
k = ln(C₁/C₂)/Δt
Volume and clearance
Vd = dose/C₀, CL = k·Vd
Loading dose
LD = Vd·C/(S·F)
Maintenance rate
R = CL·Css/(S·F)
Average steady state
Css,avg = S·F·D/(CL·τ)
Steady state
1 − (1/2)ⁿ after n half-lives; ≈ 97% after 5
Infusion
C = (R/CL)(1 − e−kt)
Interval and dose from peak and trough
τ = ln(Cpeak/Ctrough)/k, D = Vd·(Cpeak − Ctrough)
Cockcroft–Gault
CrCl = (140 − age) × kg/(72 × SCr mg/dL), × 0.85 for women
Renal adjustment
dose × CrCl/normal; Dettli Q = 1 − fe·(1 − CrCl/normal)

Method

  1. Convert the half-life to a rate constant: k = 0.693/t½.
  2. Turn per-kg values into totals: Vd (L) = L/kg × weight, CL (L/h) = L/kg/h × weight.
  3. Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
  4. For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
  5. Check the result: levels fall by half every half-life, and steady state takes about five half-lives.

2Practice problems

1.Lab values & electrolytesMedium

A patient on carbamazepine becomes confused and has a headache. Which laboratory result would most likely explain this?

  1. AA low serum magnesium
  2. BA low serum sodium
  3. CA high serum calcium
  4. DA high serum potassium
2.Lab values & electrolytesHard

A patient on a thiazide diuretic has a sodium of 129 mEq/L, a potassium of 3.2 mEq/L and feels weak. Which explanation fits best?

  1. AThe results are typical of kidney failure
  2. BThe patient has primary hyperaldosteronism with high sodium
  3. CThe patient is overhydrated from IV fluids only
  4. DThe diuretic is causing sodium and potassium loss
3.Arterial blood gas interpretationMedium
pH7.357.457.42PaCO₂mmHg354551HCO₃⁻mmol/L222632uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.42, PaCO₂ 51 mmHg (6.8 kPa) and HCO₃⁻ 32 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.

pH 7.42PaCO₂ 51 mmHg (6.8 kPa)HCO₃⁻ 32 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.31PaCO₂mmHg354529HCO₃⁻mmol/L222614added normal-gap acidosispure high-gap acidosisadded metabolic alkalosis

A practice gas shows pH 7.31, PaCO₂ 29 mmHg (3.9 kPa) and HCO₃⁻ 14 mmol/L, with Na⁺ 140 and Cl⁻ 109 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.

pH 7.31PaCO₂ 29 mmHg (3.9 kPa)HCO₃⁻ 14 mmol/LNa⁺ 140 mmol/LCl⁻ 109 mmol/L
  1. 1added normal-gap acidosis
  2. 2pure high-gap acidosis
  3. 3added metabolic alkalosis
5.Clinical pharmacokineticsMedium
110100t (h)C (mg/L)1 h10 mg/L3 h5.7 mg/L

Practice exercise (made-up values, not clinical guidance). After an IV dose of gentamicin, the level is 10 mg/L at 1 h and 5.7 mg/L at 3 h. Assume first-order elimination. Find the half-life t½.

C1 = 10 mg/L (1 h)C2 = 5.7 mg/L (3 h)
t½ = h
6.Clinical pharmacokineticsHard
61 kg59 · womanSCr 2.8 mg/dLCrCl → τ = ? h

Practice exercise (made-up values, not clinical guidance). A 59-year-old woman weighs 61 kg and has a serum creatinine of 2.8 mg/dL (248 µmol/L). Estimate the creatinine clearance with Cockcroft–Gault: CrCl (mL/min) = (140 − age) × weight (kg)/(72 × SCr in mg/dL), × 0.85 for women. The usual tobramycin regimen is 120 mg every 8 h for a creatinine clearance of 120 mL/min. Keeping the dose, the interval is lengthened in proportion to the fall in clearance. Find the new dosing interval τ. Round to one decimal place.

59 years61 kgwomanSCr = 2.8 mg/dL120 mg q8hCrCl normal 120 mL/min
τ = h
7.Clinical nursingMedium

A nurse finds a patient lying on the floor beside the bed. What should be done first?

  1. AComplete the incident report form
  2. BLift the patient straight back into bed so they are comfortable
  3. CPhone the patient's family
  4. DCheck the patient for injuries before moving them
8.Clinical nursingHard

A patient with small cell lung cancer has hyponatraemia, concentrated urine and no signs of dehydration. Which condition and first measure fit best?

  1. ADiabetes insipidus, managed with extra fluids
  2. BCushing's syndrome, managed with potassium
  3. CSIADH, often managed first with fluid restriction
  4. DDehydration, managed with an IV fluid bolus

3Solutions

1.Lab values & electrolytes

Answer: B · A low serum sodium

Why this is the answer

Carbamazepine (and oxcarbazepine) can cause hyponatremia by increasing the effect of ADH, which can cause confusion, headache and seizures. The drug can also cause serious blood disorders, so the CBC is monitored.Checked against: Syndrome of Inappropriate Antidiuretic Hormone Secretion · StatPearls (NCBI Bookshelf) (2023); Carbamazepine · StatPearls (NCBI Bookshelf) (2023)

Answer

B · A low serum sodium
2.Lab values & electrolytes

Answer: D · The diuretic is causing sodium and potassium loss

Why this is the answer

Thiazide diuretics commonly cause hyponatremia and hypokalemia, especially in older adults. The nurse reports the results, and the prescriber reviews the drug and electrolyte replacement.Checked against: Thiazide Diuretics · StatPearls (NCBI Bookshelf) (2025)

Answer

D · The diuretic is causing sodium and potassium loss
3.Arterial blood gas interpretation

Answer: fully compensated

Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.

ROME, value by value

pH 7.42: within 7.35–7.45
PaCO₂ 51 mmHg: high (35–45)
HCO₃⁻ 32 mmol/L: high (22–26)

Compensation

Clinically, also check the expected compensation: for a metabolic alkalosis PaCO₂ ≈ 0.7·HCO₃⁻ + 21 = 41.4–45.4 mmHg, so the measured 51 mmHg points to an additional respiratory acidosis (a mixed disorder). The answer above is the ROME exam classification.

fully compensated

Answer

fully compensated
4.Arterial blood gas interpretation

Answer: added normal-gap acidosis

Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.

Anion gap

AG = 140 − (109 + 14) = 17

Delta ratio

(17 − 12)/(24 − 14) = 0.5
added normal-gap acidosis

Answer

added normal-gap acidosis
5.Clinical pharmacokinetics

Answer: t½ = 2.466 h

Clearance and volume: at steady state the dosing rate equals the elimination rate, so the maintenance rate = CL × Css. A loading dose is LD = Vd × C/(S × F), with S the salt factor (fraction of active drug in the salt) and F the bioavailability (1 for IV). From two levels, k = ln(C₁/C₂)/Δt; for an IV bolus Vd = dose/C₀ and CL = k × Vd. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Elimination rate constant

k = ln(10/5.7)/2 = 0.5621/2 = 0.2811 h⁻¹

Half-life

t½ = 0.693/0.2811 = 2.466 h

Answer

t½ = 2.466 h
6.Clinical pharmacokinetics

Answer: τ = 46.1 h

Designing a regimen: with rapid IV doses the level falls from peak to trough during the interval, so τ = ln(Cpeak/Ctrough)/k and the dose that restores the peak is Vd × (Cpeak − Ctrough). The average steady-state level is Css,avg = S·F·dose/(CL·τ). An infusion rises as C = (R/CL)(1 − e−kt). For a drug cleared by the kidneys, the dose is scaled with the creatinine clearance. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Creatinine clearance (Cockcroft–Gault)

CrCl = (140 − 59)·61/(72·2.8)·0.85 = 20.83 mL/min

New interval

τ = 8·120/20.83 = 46.08 ≈ 46.1 h

Answer

τ = 46.1 h

In practice the interval is rounded to a convenient value such as 12, 24, 36 or 48 h.

7.Clinical nursing

Answer: D · Check the patient for injuries before moving them

Why this is the answer

After a fall, the patient is assessed first, including airway, breathing, circulation, head injury and possible fractures, before deciding how to move them safely. The fall is then reported and the care plan reviewed.

Answer

D · Check the patient for injuries before moving them
8.Clinical nursing

Answer: C · SIADH, often managed first with fluid restriction

Why this is the answer

In SIADH, excess ADH makes the kidneys retain water, so blood sodium is diluted while urine is concentrated. Small cell lung cancer is a classic cause; fluid restriction is a common first step.

Answer

C · SIADH, often managed first with fluid restriction

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