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

When should the nurse check the glucose before giving a mealtime dose of rapid-acting insulin (such as lispro)?

  1. AOnly once a day, then giving the same dose before every meal
  2. BJust before the meal, giving insulin when the food is ready
  3. CTwo hours after the meal, then giving the insulin dose
  4. DAt bedtime only, then giving the mealtime insulin overnight
2.Lab values & electrolytesHard

A patient with no kidney problems and a normal ECG has a potassium of 6.2 mEq/L. The laboratory comments that the sample was hemolyzed. What is the best interpretation?

  1. AThe potassium is certainly normal and needs no follow-up
  2. BThe patient has kidney failure
  3. CThe result may be falsely high; assess the patient and arrange a repeat sample
  4. DTreat immediately with insulin and glucose without further checks
3.Arterial blood gas interpretationMedium
pH7.357.457.15PaCO₂mmHg354518HCO₃⁻mmol/L22266uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.15, PaCO₂ 18 mmHg (2.4 kPa) and HCO₃⁻ 6 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.

pH 7.15PaCO₂ 18 mmHg (2.4 kPa)HCO₃⁻ 6 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.16PaCO₂mmHg354549HCO₃⁻mmol/L222617appropriate compensationadded respiratory acidosisadded respiratory alkalosis

A practice gas shows a metabolic acidosis: pH 7.16, PaCO₂ 49 mmHg (6.5 kPa), HCO₃⁻ 17 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.

pH 7.16PaCO₂ 49 mmHg (6.5 kPa)HCO₃⁻ 17 mmol/L
  1. 1appropriate compensation
  2. 2added respiratory acidosis
  3. 3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
VCss​ = 4 mg/LR = ? mg/minCL = 800 mL/min

Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 800 mL/min. What infusion rate R in mg/min keeps the steady-state level at 4 mg/L?

CL = 800 mL/minCss = 4 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
95 kg73 · womanSCr 2.1 mg/dLCrCl → D = ? mg

Practice exercise (made-up values, not clinical guidance). A 73-year-old woman weighs 95 kg and has a serum creatinine of 2.1 mg/dL (186 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 gentamicin dose is 120 mg every 8 h for a creatinine clearance of 120 mL/min. Keeping the interval, the dose is reduced in proportion to the creatinine clearance. Find the adjusted dose D. Round to the nearest whole number.

73 years95 kgwomanSCr = 2.1 mg/dL120 mg q8hCrCl normal 120 mL/min
D = mg
7.Clinical nursingMedium

Why are patients advised to rinse their mouth after using an inhaled corticosteroid?

  1. ATo prevent tooth decay from the sugar in it
  2. BTo reduce the risk of oral thrush
  3. CTo make the drug work faster
  4. DTo stop the drug from being absorbed in the lungs
8.Clinical nursingHard

A patient's urine output has been very low for 12 hours and creatinine has risen sharply. Which drug on the chart is most likely to need review?

  1. AParacetamol at usual doses
  2. BA non-steroidal anti-inflammatory drug such as ibuprofen
  3. CA vitamin supplement
  4. DAn emollient cream

3Solutions

1.Lab values & electrolytes

Answer: B · Just before the meal, giving insulin when the food is ready

Why this is the answer

Rapid-acting insulin starts working within about 15 minutes, so glucose is checked before the meal and the insulin is given when the meal is available, usually within 15 minutes of eating. Giving it without food can cause hypoglycemia.

Answer

B · Just before the meal, giving insulin when the food is ready
2.Lab values & electrolytes

Answer: C · The result may be falsely high; assess the patient and arrange a repeat sample

Why this is the answer

Red cells are rich in potassium. When they break in the tube (from a tight tourniquet, fist clenching or a small needle), potassium leaks out and the result is falsely high. The nurse still assesses the patient and repeats the test, because true hyperkalemia must not be missed.Checked against: Potassium Blood Test · MedlinePlus (NIH) (2024)

Answer

C · The result may be falsely high; assess the patient and arrange a repeat sample
3.Arterial blood gas interpretation

Answer: partially 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 7.15: acidaemia (below 7.35)
PaCO₂ 18 mmHg: low (35–45)
HCO₃⁻ 6 mmol/L: low (22–26)

Compensation

partially compensated

Answer

partially compensated
4.Arterial blood gas interpretation

Answer: added respiratory 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.

Expected PaCO₂

1.5·17 + 8 = 33.5 → 31.5–35.5 mmHg

Compare

measured: 49 mmHg → added respiratory acidosis

Answer

added respiratory acidosis
5.Clinical pharmacokinetics

Answer: R = 3.2 mg/min

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.

Clearance in L/min

800 mL/min = 0.8 L/min

Infusion rate

R = 0.8·4 = 3.2 mg/min

Answer

R = 3.2 mg/min
6.Clinical pharmacokinetics

Answer: D = 36 mg

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 − 73)·95/(72·2.1)·0.85 = 35.78 mL/min

Adjusted dose

D = 120·35.78/120 = 35.78 ≈ 36 mg

Answer

D = 36 mg

In practice the dose is rounded to a convenient amount and checked against the product information.

7.Clinical nursing

Answer: B · To reduce the risk of oral thrush

Why this is the answer

Inhaled steroid deposited in the mouth and throat can cause oral candidiasis and a hoarse voice. Rinsing and using a spacer reduce this.

Answer

B · To reduce the risk of oral thrush
8.Clinical nursing

Answer: B · A non-steroidal anti-inflammatory drug such as ibuprofen

Why this is the answer

In acute kidney injury, nephrotoxic drugs (such as NSAIDs and aminoglycosides) and drugs that reduce renal perfusion (such as ACE inhibitors and diuretics) are reviewed and often stopped.

Answer

B · A non-steroidal anti-inflammatory drug such as ibuprofen

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