Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
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.
In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
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
Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
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 macrocytic anemia with numbness and tingling of the feet and poor balance. Which deficiency is most likely?
AFolate (folic acid)
BIron (low ferritin)
CVitamin C (ascorbic acid)
DVitamin B12
2.Lab values & electrolytesHard
A 5-year-old has new bruising and petechiae two weeks after a viral illness. Platelets are 12 × 10³/µL; hemoglobin, white cell count and clotting times are normal. Which condition is most likely?
ADisseminated intravascular coagulation
BHemophilia A
CAcute leukemia
DImmune thrombocytopenia (ITP)
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.31, PaCO₂ 74 mmHg (9.9 kPa) and HCO₃⁻ 36 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.31PaCO₂ 74 mmHg (9.9 kPa)HCO₃⁻ 36 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 132, Cl⁻ 86 and HCO₃⁻ 18 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 132 mmol/LCl⁻ 86 mmol/LHCO₃⁻ 18 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of gentamicin, the level is 9 mg/L at 1 h and 3.6 mg/L at 5 h. Assume first-order elimination. Find the half-life t½.
C1 = 9 mg/L (1 h)C2 = 3.6 mg/L (5 h)
t½ = h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). An 84 kg patient on regular tobramycin doses is at steady state. The target is a peak of 7 mg/L and a trough of 1 mg/L, and the volume of distribution is 0.3 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?
84 kgVd = 0.3 L/kgCpeak = 7 mg/LCtrough = 1 mg/L
D = mg
7.Clinical nursingMedium
A patient is started on spironolactone while already taking an ACE inhibitor. Which blood result should be watched most closely?
ASodium, because both drugs raise it sharply
BCalcium, because both drugs lower it
CGlucose, because both drugs cause hyperglycaemia
DPotassium, because both drugs can raise it
8.Clinical nursingHard
The fluid in an underwater seal chest drain has stopped swinging with breathing. What could this mean?
AThe drain is working perfectly and needs no checks
BThe water seal has been set up correctly for the first time
CThe patient's breathing has become normal and the drain can be pulled out by the nurse
DThe tube may be kinked or blocked, or the lung may have fully re-expanded
3Solutions
1.Lab values & electrolytes
Answer: D · Vitamin B12
Why this is the answer
B12 and folate deficiencies both cause macrocytic anemia, but B12 deficiency also damages the spinal cord and peripheral nerves, causing numbness, tingling and poor balance. Neurological damage can become permanent if treatment is delayed.Checked against:Vitamin B12 deficiency anemia - Medical Encyclopedia · MedlinePlus (NIH) (2026); Vitamin B12 level - Medical Encyclopedia · MedlinePlus (NIH) (2026)
Answer
D · Vitamin B12
2.Lab values & electrolytes
Answer: D · Immune thrombocytopenia (ITP)
Why this is the answer
ITP is immune destruction of platelets, often after a viral infection in children. It causes isolated thrombocytopenia with normal other counts and clotting tests. Leukemia usually affects other cell lines, DIC prolongs clotting times and hemophilia prolongs the aPTT.Checked against:Immune thrombocytopenic purpura (ITP) · MedlinePlus (NIH) (2026)
Answer
D · Immune thrombocytopenia (ITP)
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 7.31: acidaemia (below 7.35)
PaCO₂ 74 mmHg: high (35–45)
HCO₃⁻ 36 mmol/L: high (22–26)
Compensation
partially compensated
Answer
partially compensated
4.Arterial blood gas interpretation
Answer: delta ratio = 2.667
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 = 132 − (86 + 18) = 28
Delta ratio
(28 − 12)/(24 − 18) = 16/6 = 2.667
Answer
delta ratio = 2.667
Above 2: an added metabolic alkalosis.
5.Clinical pharmacokinetics
Answer: t½ = 3.026 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(9/3.6)/4 = 0.9163/4 = 0.2291 h⁻¹
Half-life
t½ = 0.693/0.2291 = 3.026 h
Answer
t½ = 3.026 h
6.Clinical pharmacokinetics
Answer: D = 151.2 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.
Volume of distribution
Vd = 0.3·84 = 25.2 L
Dose
D = 25.2·(7 − 1) = 151.2 mg
Answer
D = 151.2 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: D · Potassium, because both drugs can raise it
Why this is the answer
Spironolactone and ACE inhibitors both reduce potassium excretion, so the combination can cause dangerous hyperkalaemia. Potassium and renal function should be checked regularly.
Answer
D · Potassium, because both drugs can raise it
8.Clinical nursing
Answer: D · The tube may be kinked or blocked, or the lung may have fully re-expanded
Why this is the answer
Loss of swing can mean a kinked, clamped or blocked tube, or re-expansion of the lung. The nurse checks the tubing and the patient and reports the finding.
Answer
D · The tube may be kinked or blocked, or the lung may have fully re-expanded