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 with diabetes is found unconscious with a blood glucose of 38 mg/dL (2.1 mmol/L). Which treatment is appropriate?

  1. AGlucose tablets placed between the patient's teeth
  2. BA dose of rapid-acting insulin to stabilize the glucose
  3. CGlucagon by injection, or IV dextrose as prescribed
  4. DOrange juice poured slowly into the patient's mouth
2.Lab values & electrolytesHard

A woman with preeclampsia is receiving IV magnesium sulfate. Her urine output has fallen and her creatinine is rising. Why is this a concern?

  1. AMagnesium is broken down by the liver, so it will stop working soon
  2. BMagnesium is excreted by the kidneys, so it can build up to toxic levels
  3. CPoor kidney function makes the magnesium level fall quickly
  4. DMagnesium causes potassium loss, so the kidneys release more fluid
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 8 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 8 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
pH7.357.457.27PaCO₂mmHg354525HCO₃⁻mmol/L222611added normal-gap acidosispure high-gap acidosisadded metabolic alkalosis

A practice gas shows pH 7.27, PaCO₂ 25 mmHg (3.3 kPa) and HCO₃⁻ 11 mmol/L, with Na⁺ 134 and Cl⁻ 94 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.

pH 7.27PaCO₂ 25 mmHg (3.3 kPa)HCO₃⁻ 11 mmol/LNa⁺ 134 mmol/LCl⁻ 94 mmol/L
  1. 1added normal-gap acidosis
  2. 2pure high-gap acidosis
  3. 3added metabolic alkalosis
5.Clinical pharmacokineticsMedium
Vd = 0.7 L/kgC = 20 mg/LLD = ? mg57 kg · S = 0.92 · F = 1

Practice exercise (made-up values, not clinical guidance). A 57 kg patient is to be loaded intravenously with phenytoin sodium to reach a phenytoin level of 20 mg/L. The volume of distribution is 0.7 L/kg and the salt factor is S = 0.92. Find the loading dose LD of phenytoin sodium in mg.

57 kgVd = 0.7 L/kgC = 20 mg/LS = 0.92
LD = mg
6.Clinical pharmacokineticsHard
VCss,avg​ = ?1500 mg · q12hCL = 5 L/hS = 1 · F = 1

Practice exercise (made-up values, not clinical guidance). A patient receives vancomycin IV every 12 h; the vancomycin clearance is 5 L/h. Each dose is 1500 mg of vancomycin. Find the average steady-state level Css,avg.

CL = 5 L/hτ = 12 hS = 1F = 1D = 1500 mg
Css,avg = mg/L
7.Clinical nursingMedium

A patient with hyponatraemia should be especially monitored for:

  1. AIncreased hair growth
  2. BBright red rash
  3. CHearing loss
  4. DConfusion, headache and seizures
8.Clinical nursingHard

A catheterised postoperative patient has passed only 10 mL of urine in the last two hours. What should the nurse check first?

  1. AThe patient's weight from last year
  2. BThe patient's cholesterol level from the admission bloods
  3. CThat the catheter and tubing are not kinked or blocked
  4. DWhether the patient has eaten breakfast

3Solutions

1.Lab values & electrolytes

Answer: C · Glucagon by injection, or IV dextrose as prescribed

Why this is the answer

A patient who cannot swallow safely must not receive anything by mouth because of the risk of aspiration. Glucagon (injected or intranasal) or IV dextrose raises the glucose; once awake and able to swallow, the patient is given oral carbohydrate and a snack.Checked against: Hypoglycemia · MedlinePlus (NIH); Low blood sugar · MedlinePlus (NIH) (2026)

Answer

C · Glucagon by injection, or IV dextrose as prescribed
2.Lab values & electrolytes

Answer: B · Magnesium is excreted by the kidneys, so it can build up to toxic levels

Why this is the answer

Magnesium is cleared by the kidneys, so falling urine output and rising creatinine raise the risk of toxicity. The nurse checks deep tendon reflexes, respiratory rate and level of consciousness, reports the findings and keeps calcium gluconate available as the antidote.Checked against: Magnesium Toxicity (StatPearls) · StatPearls (NCBI Bookshelf) (2026)

Answer

B · Magnesium is excreted by the kidneys, so it can build up to toxic levels
3.Arterial blood gas interpretation

Answer: PaCO₂ = 20 mmHg

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.

Winter’s formula

PaCO₂ = 1.5·8 + 8 = 20 mmHg
range: 18–22 mmHg

Answer

PaCO₂ = 20 mmHg
4.Arterial blood gas interpretation

Answer: pure high-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 = 134 − (94 + 11) = 29

Delta ratio

(29 − 12)/(24 − 11) = 1.308
pure high-gap acidosis

Answer

pure high-gap acidosis
5.Clinical pharmacokinetics

Answer: LD = 867.4 mg

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.

Volume of distribution

Vd = 0.7·57 = 39.9 L

Amount of active drug needed

Vd·C = 39.9·20 = 798 mg

Loading dose

LD = 798/(0.92·1) = 867.4 mg

Answer

LD = 867.4 mg

In practice the dose is rounded to a convenient amount.

6.Clinical pharmacokinetics

Answer: Css,avg = 25 mg/L

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.

Average steady-state level

Css,avg = S·F·D/(CL·τ) = 1·1·1500/(5·12) = 25 mg/L

Answer

Css,avg = 25 mg/L
7.Clinical nursing

Answer: D · Confusion, headache and seizures

Why this is the answer

Low sodium causes water to move into brain cells, leading to nausea, headache, confusion and in severe cases seizures. It must be corrected slowly to avoid osmotic demyelination.

Answer

D · Confusion, headache and seizures
8.Clinical nursing

Answer: C · That the catheter and tubing are not kinked or blocked

Why this is the answer

A sudden fall in catheter output may be due to a kinked, clamped or blocked catheter. If it is draining freely, the nurse assesses fluid status and vital signs and escalates possible hypovolaemia or AKI.

Answer

C · That the catheter and tubing are not kinked or blocked

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