Q2 LONG ANSWER 10M ★ High-yield

Describe the laboratory diagnosis of bloodstream infection. Add a note on pyrexia of unknown origin.

Microbiology · 2nd Professional — Para-Clinical

Definitions

Bacteraemia is the presence of viable bacteria in the blood; sepsis is life-threatening organ dysfunction caused by a dysregulated host response to infection; and septic shock is sepsis with circulatory and metabolic abnormality carrying a substantially higher mortality
Transient bacteraemia follows dental procedures, endoscopy and even tooth-brushing, and is cleared within minutes; intermittent bacteraemia occurs with an abscess; and continuous bacteraemia indicates an intravascular source — endocarditis, an infected line or a mycotic aneurysm
The pattern matters practically: because endocarditis produces continuous bacteraemia, blood may be drawn at any time, and repeatedly positive cultures with the same organism are themselves evidence for it

Blood Culture Technique

Element	Requirement and reason
Volume — the single most important variable	Bacteraemia is often of low density (fewer than 10 organisms per mL in adults, and sometimes fewer than 1). 8 to 10 mL per bottle and 20 to 30 mL per set in adults; yield rises by about 3% for every additional mL. In neonates and children, 1 to 3 mL according to weight, since their bacteraemia is of higher density
Number of sets	Two to three sets from separate venepuncture sites. Two sets detect about 90% and three about 98% of bacteraemias. A single set is inadequate and, crucially, makes it impossible to distinguish contamination from true infection
Timing	Draw at any time; it is not necessary to wait for a fever spike, and doing so wastes hours. In suspected endocarditis, three sets are taken over a period before starting treatment where the patient is stable
Before antibiotics	Absolutely; a single prior dose substantially reduces yield
Skin antisepsis	The main determinant of contamination rates. 2% chlorhexidine in alcohol (or povidone-iodine), applied with friction and allowed TO dry fully — the drying time is the contact time, and not waiting is the commonest error. Do not palpate the vein again after preparing the skin
Bottle preparation	Disinfect the rubber septum with alcohol; it is not sterile under the cap
Blood-to-broth ratio	1:5 to 1:10, to dilute out serum bactericidal factors, complement and any antibiotic present; SPS in the bottle also neutralises these
Site	Peripheral venepuncture. Do not draw from an existing line, which has a much higher contamination rate — the exception is suspected CLABSI, where a paired line and peripheral sample is deliberately taken

CLINICAL PEARL

Volume matters more than anything else, and a single small bottle is close to useless. An adult with S. Aureus bacteraemia may have fewer than one organism per millilitre of blood. Drawing 3 mL when 10 is required does not give a slightly less sensitive test — it may sample no organisms at all. Every other refinement of blood culture technique is secondary to filling the bottles properly.

Processing and Interpretation

Automated continuous-monitoring systems (BacT/alert, BACTEC) detect CO2 production colorimetrically or fluorimetrically every few minutes, flagging positives within hours rather than at daily inspection. Most significant pathogens are detected within 24 to 48 hours; bottles are held for 5 days, and longer for fastidious organisms such as the HACEK group, Brucella and fungi
On a positive flag — an immediate gram stain is performed and telephoned to the clinician at once, since it narrows the empirical treatment within minutes; subculture follows, with identification and susceptibility testing, and MALDI-TOF or molecular panels applied directly to the positive broth can give an identification within an hour
Special methods — lysis-centrifugation for fungi and mycobacteria; biphasic (Castaneda) bottles for Brucella; and bone marrow culture, which is the most sensitive test in enteric fever and remains positive despite prior antibiotics

Finding	Interpretation
The same organism in multiple sets	True bacteraemia
S. Aureus, S. Pneumoniae, E. Coli and other Enterobacterales, Pseudomonas, Candida, group A streptococcus	Almost always significant, even from a single bottle — never dismissed as contaminants
Coagulase-negative staphylococci, Bacillus, Corynebacterium, Cutibacterium, viridans streptococci from a single bottle	Usually contaminants — skin flora introduced during collection. But they are genuine pathogens in the presence of a prosthetic valve, joint or line, so the clinical context decides
Growth after a long delay, or in only one of a pair	Favours contamination
Polymicrobial growth	Suggests gut or biliary source, or contamination
A contamination rate above 3%	Indicates a problem with collection technique, and is itself an audit measure — each false positive costs unnecessary antibiotics, extra tests and prolonged stay

Pyrexia of Unknown Origin

Pyrexia OF unknown origin (PUO) is a temperature above 38.3°C on several occasions, lasting more than 3 weeks, with no diagnosis after appropriate initial investigation.

The 3-week criterion excludes self-limiting viral illness, and the "appropriate investigation" criterion excludes fevers that are simply uninvestigated — both are what make the definition useful
Causes in India — infections predominate (tuberculosis, especially extrapulmonary and miliary; enteric fever; abscesses; infective endocarditis; malaria; kala-azar; brucellosis; HIV and its opportunistic infections); then malignancy (lymphoma, leukaemia, renal cell carcinoma); connective tissue and inflammatory disease (adult Still disease, vasculitis, systemic lupus); and miscellaneous causes including drug fever, which is common and easily reversed
Tuberculosis is much the commonest cause of PUO in India, and extrapulmonary disease with a normal chest radiograph accounts for most of the diagnostic difficulty
The approach is a repeated history and examination — travel, animal and occupational exposure, drugs, sexual history, surgery, prostheses, and a careful search for lymphadenopathy, murmurs, skin lesions and fundal changes. Re-examination often yields what the first examination missed
Investigations — repeated blood cultures; malaria films and rapid tests; tuberculosis workup with sputum, CBNAAT and tissue; HIV testing; serology directed by exposure; imaging including ultrasound, CT and where available FDG-pet; bone marrow aspiration and culture, which is high-yield in India for tuberculosis, kala-azar, histoplasmosis and haematological malignancy; and biopsy of any accessible abnormality
Avoid empirical "therapeutic trials" where possible — empirical antitubercular treatment is very commonly given in India and obscures the diagnosis for months while committing the patient to hepatotoxic drugs; where it is unavoidable, the endpoint should be defined in advance
Stop unnecessary drugs; drug fever resolves within 72 hours of withdrawal and is diagnosed no other way

Laboratory Diagnosis of Enteric Fever

Week of illness	Test with the highest yield	Comment
Week 1	Blood culture (about 80% positive)	The definitive test; 10 to 15 mL of blood, since the bacteraemia is of low density. Yield falls sharply after antibiotics
Any week, especially after antibiotics	Bone marrow culture (about 90%, the highest of all)	Remains positive despite prior treatment, because antibiotic concentrations in marrow are lower; invasive, so reserved for difficult cases
Week 2 onward	WIDAL test (agglutinins to O and H antigens)	A rising titre in paired sera is what matters; a single titre is unreliable in an endemic population where baseline titres are high from past infection and vaccination. Negative in week 1, so it is frequently ordered too early and then wrongly reassures
Week 2 to 3	Stool and urine culture	Positive as excretion begins; also identifies carriers
Chronic carriage	Repeated stool culture; bile or duodenal aspirate; Vi antibody	The gallbladder is the reservoir, particularly with gallstones
Any stage	Typhidot (IgM), rapid antigen tests, PCR	Convenient but of variable performance; they do not replace culture

The Widal test is over-used and over-interpreted in India — ordered in the first week when it is necessarily negative, and acted upon as a single titre when only a rising titre is meaningful. A blood culture in the first week answers the question properly and also gives susceptibilities, which matter given widespread resistance

Applied Aspects

Fill the bottles — inadequate volume is the commonest reason a blood culture is negative in a genuinely bacteraemic patient
Take two or three sets from separate sites, so that a coagulase-negative staphylococcus can be interpreted at all; a single set makes the result uninterpretable whichever way it falls
Let the antiseptic dry before puncturing; this is the commonest cause of a high contamination rate and takes 30 seconds
Never dismiss S. Aureus in a blood culture — it is essentially never a contaminant, requires a search for endocarditis and metastatic foci, echocardiography, and at least 14 days of treatment
Do not wait for a fever spike; this is a persistent myth that delays diagnosis and treatment by hours
Consider bone marrow culture in prolonged fever in India; it is high-yield for enteric fever, tuberculosis and kala-azar together, on a single specimen
Do not order a Widal in the first week; it is necessarily negative and the negative result is then wrongly taken to exclude enteric fever
A single Widal titre means little in an endemic population; only a rising titre in paired sera is interpretable, and the test is a poor substitute for a blood culture
Re-examine the patient in a PUO; a new murmur, a node, a skin lesion or a fundal change appearing over days is frequently what makes the diagnosis when investigation has not
Review the drug chart in every prolonged fever; drug fever is common, costs nothing to test for, and resolves within 72 hours of stopping the agent
Avoid an empirical antitubercular trial where the diagnosis can still be pursued; it is common in India, obscures the picture for months and commits the patient to hepatotoxic drugs

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