Joint aspiration — formative SAQs

SAQ 1 (10 marks)

A 3-year-old boy is brought to the emergency department refusing to walk. He is febrile at 39.1°C, holds the right hip flexed and abducted, and cries when the hip is moved. The C-reactive protein is 95 mg per litre, the white cell count is 16,000 per microlitre, and an ultrasound shows a right hip effusion. The team is planning an ultrasound-guided hip aspiration. [1] [2]

1. Score this child's risk of septic arthritis using the Kocher and Caird prediction rule, and state what the score dictates about the next step. (3) [1]
2. Outline the structured pre-aspiration assessment and the relative contraindications you must exclude before draping. (4) [2] [5]
3. Describe the synovial fluid tests you will request, and give the synovial white cell count threshold that most strongly supports septic arthritis. (3) [3] [5]

Model answer — SAQ 1

(1) Prediction score and what it dictates (3). The Kocher predictors are a fever above 38.5°C, refusal to bear weight, an erythrocyte sedimentation rate above 40 mm per hour and a peripheral white cell count above 12,000 per microlitre; the Caird refinement adds a C-reactive protein above 20 mg per litre. This child has fever, refusal to bear weight, a raised white cell count and a markedly raised CRP, meeting four of the five criteria. With this many predictors the probability of septic arthritis is high, and the score dictates that this is septic arthritis until proven otherwise: I arrange an ultrasound-guided hip aspiration to obtain synovial fluid, take a blood culture, start empirical intravenous antibiotics, and involve the orthopaedic team for urgent washout. I would not observe this child. [1]

(2) Pre-aspiration assessment and relative contraindications (4). I perform a structured pre-aspiration check. I examine the joint for swelling, warmth, erythema and the pain at the limit of passive movement, and I assess the child as a whole for fever, toxicity and shock, and the distal neurovascular status of the limb. I score the prediction rule as above. I check the overlying skin specifically for cellulitis or a wound, because passing the needle through infected skin seeds the joint and causes iatrogenic septic arthritis — if cellulitis overlies the planned approach I choose an uninfected approach or defer. I exclude a bleeding risk by asking about haemophilia, anticoagulants and thrombocytopenia, and by checking a platelet count and coagulation where relevant, because aspirating an uncorrected coagulopathic child produces a haemarthrosis. I confirm this is a deep hip that must be aspirated under ultrasound, I obtain and document consent, and I plan the analgesia and sedation. [2] [5]

(3) Synovial fluid tests and the threshold (3). I send the aspirate for a white cell count and differential, a Gram stain, and culture; in the young child I add a Kingella kingae-specific culture or polymerase chain reaction, and I consider crystals or a viral or mycobacterial study if the picture is atypical. The synovial white cell count threshold that most strongly supports septic arthritis is above 50,000 per microlitre with a neutrophil predominance, and counts are frequently above 100,000; however, I would emphasise that a count below 50,000 does not exclude sepsis in a partially treated child or in Kingella disease, so the result is read with the whole clinical picture. [3] [5]

SAQ 2 (10 marks)

A 6-year-old with known severe haemophilia A presents with a swollen, painful right knee after a minor fall. The joint is warm and tense but he is afebrile and systemically well. The emergency team plans to aspirate the knee "to exclude sepsis". [4] [5]

1. Explain why routine aspiration of this joint is the wrong first step, and what should happen instead. (4) [5]
2. Under what specific circumstances would aspiration be justified, and how must it be performed safely? (3) [5]
3. Contrast the synovial fluid picture of this likely haemarthrosis with that of septic arthritis. (3) [4]

Model answer — SAQ 2

(1) Why aspiration is the wrong first step (4). The working diagnosis in a boy with severe haemophilia and a swollen painful joint after trauma is a haemarthrosis, not sepsis — he is afebrile and systemically well, which lowers the probability of a septic joint. The first step is factor replacement with the haematology team, rest and analgesia, not a needle into the joint. Passing a needle into an uncorrected haemophilic joint worsens the bleed, can raise intra-articular pressure enough to compromise the limb, and converts a manageable haemarthrosis into a surgical emergency. The right sequence is therefore factor cover, rest and observation, with aspiration reserved only if there is genuine diagnostic doubt about sepsis. [5]

(2) When aspiration is justified and how it is done safely (3). Aspiration is justified only when there is a real clinical suspicion of septic arthritis on top of the haemophilia — for example a fever, a rapidly worsening joint despite factor cover, or a markedly raised inflammatory response. Even then it is performed only after adequate factor replacement to a level agreed with haematology, by an experienced operator using sterile technique, and with the joint observed afterwards for a re-bleed. The child is not aspirated on a routine "exclude sepsis" basis when the clinical picture is clearly a traumatic haemarthrosis. [5]

(3) Haemarthrosis versus septic arthritis on the fluid (3). A haemarthrosis yields a uniformly bloody aspirate that typically clots, with red cells dominating and a white cell count that is modest and roughly proportionate to the blood contamination. A septic joint yields a turbid, yellow-green, often non-clotting fluid with a white cell count above 50,000 per microlitre, a neutrophil predominance, and a Gram stain that may be positive. The distinction matters because a bloody aspirate from trauma is managed by factor cover and rest, whereas a turbid high-count aspirate is managed with antibiotics and orthopaedic washout. [4]