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 has a sodium of 124 mEq/L, a headache and new confusion. Which nursing measure is most appropriate?

  1. AGive the next dose of the prescribed thiazide diuretic early
  2. BPut seizure precautions in place and monitor neurological status
  3. CKeep the patient flat and limit neurological checks to once a shift
  4. DEncourage the patient to drink plenty of plain water
2.Lab values & electrolytesHard

A patient has a positive hepatitis C antibody (anti-HCV) test. What is the most appropriate next step?

  1. ATell the patient they definitely have chronic hepatitis C
  2. BTest for HCV RNA to find out whether infection is current
  3. CGive hepatitis C vaccine to prevent further infection
  4. DRepeat the antibody test, as a positive test is always false
3.Arterial blood gas interpretationMedium
pH7.357.457.37PaCO₂mmHg354525HCO₃⁻mmol/L222614uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.37, PaCO₂ 25 mmHg (3.3 kPa) and HCO₃⁻ 14 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.

pH 7.37PaCO₂ 25 mmHg (3.3 kPa)HCO₃⁻ 14 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.19PaCO₂mmHg354519HCO₃⁻mmol/L22267appropriate compensationadded respiratory acidosisadded respiratory alkalosis

A practice gas shows a metabolic acidosis: pH 7.19, PaCO₂ 19 mmHg (2.5 kPa), HCO₃⁻ 7 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.

pH 7.19PaCO₂ 19 mmHg (2.5 kPa)HCO₃⁻ 7 mmol/L
  1. 1appropriate compensation
  2. 2added respiratory acidosis
  3. 3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
VCss​ = 10 mg/LR = ? mg/h · S = 0.8CL = 0.035 L/kg/h70 kg

Practice exercise (made-up values, not clinical guidance). A 70 kg patient is to receive an aminophylline infusion (salt factor S = 0.8: 80% of aminophylline is theophylline). The theophylline clearance is 0.035 L/kg/h. What aminophylline infusion rate R in mg/h gives a steady-state theophylline level of 10 mg/L?

70 kgCL = 0.035 L/kg/hCss = 10 mg/LS = 0.8
R = mg/h
6.Clinical pharmacokineticsHard
Vd = 7 L/kg0.6 → 1 mcg/LLD = ? mcg95 kg · S = 1 · F = 0.7

Practice exercise (made-up values, not clinical guidance). A 95 kg patient has a digoxin level of 0.6 mcg/L; the target is 1 mcg/L. The volume of distribution is 7 L/kg and the tablets have F = 0.7. What extra loading dose LD raises the level to the target?

95 kgVd = 7 L/kgC = 0.6 → 1 mcg/LF = 0.7
LD = mcg
7.Clinical nursingMedium

Why is lactulose given to a patient with hepatic encephalopathy?

  1. ATo stop bleeding from varices
  2. BTo increase the liver's production of albumin
  3. CTo reduce the absorption of ammonia from the gut
  4. DTo lower the blood glucose
8.Clinical nursingHard

Which drug group is first-line for a prolonged convulsive seizure?

  1. ABenzodiazepines, such as buccal midazolam or IV lorazepam
  2. BBeta-blockers, such as bisoprolol or atenolol
  3. CAntihistamines, such as chlorphenamine
  4. DOpioids, such as morphine or diamorphine, given intravenously

3Solutions

1.Lab values & electrolytes

Answer: B · Put seizure precautions in place and monitor neurological status

Why this is the answer

Hyponatremia can cause cerebral edema and seizures. The nurse monitors neurological status, takes seizure precautions and reports the result. Extra water and thiazides can lower the sodium further.

Answer

B · Put seizure precautions in place and monitor neurological status
2.Lab values & electrolytes

Answer: B · Test for HCV RNA to find out whether infection is current

Why this is the answer

Anti-HCV shows exposure, but some people clear the virus and keep antibodies. An HCV RNA test shows whether the infection is current. There is no vaccine for hepatitis C, but antiviral treatment can cure most infections.

Answer

B · Test for HCV RNA to find out whether infection is current
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 7.37: within 7.35–7.45
PaCO₂ 25 mmHg: low (35–45)
HCO₃⁻ 14 mmol/L: low (22–26)

Compensation

Clinically, also check the expected compensation: Winter’s formula expects PaCO₂ 27–31 mmHg, so the measured 25 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.

fully compensated

Answer

fully compensated
4.Arterial blood gas interpretation

Answer: appropriate compensation

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

1.5·7 + 8 = 18.5 → 16.5–20.5 mmHg

Compare

measured: 19 mmHg → appropriate compensation

Answer

appropriate compensation
5.Clinical pharmacokinetics

Answer: R = 30.63 mg/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.

Clearance

CL = 0.035·70 = 2.45 L/h

Infusion rate

R = CL·Css/S = 2.45·10/0.8 = 30.63 mg/h

Answer

R = 30.63 mg/h
6.Clinical pharmacokinetics

Answer: LD = 380 mcg

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.

Volume of distribution

Vd = 7·95 = 665 L

Rise needed

1 − 0.6 = 0.4 mcg/L

Loading dose

LD = 665·0.4/(1·0.7) = 380 mcg

Answer

LD = 380 mcg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: C · To reduce the absorption of ammonia from the gut

Why this is the answer

Lactulose acidifies the colon and speeds transit, reducing ammonia absorption. The dose is usually adjusted to give two to three soft stools a day.

Answer

C · To reduce the absorption of ammonia from the gut
8.Clinical nursing

Answer: A · Benzodiazepines, such as buccal midazolam or IV lorazepam

Why this is the answer

Benzodiazepines enhance GABA and are first-line for seizures lasting over five minutes. If seizures continue, second-line antiepileptic drugs are given and senior help is needed.

Answer

A · Benzodiazepines, such as buccal midazolam or IV lorazepam

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