Learn — Clinical pathology: specimens, urine, CSF & laboratory values

3 notes · 3 questions on the topic

Specimen collection and transport — blood, urine, stool, sputum and CSF

Specimen collection is the set of steps by which a sample of blood, urine, stool, sputum, cerebrospinal fluid, pus or tissue is taken from the right patient, in the right container, at the right time, in the right amount, labelled at the bedside and delivered to the laboratory within the time and at the temperature in which the result still means something. Most laboratory error happens before the sample reaches the laboratory — the wrong tube, the haemolysed draw, the unlabelled bottle, the swab that sat for a night — and every one of those errors is a nursing act, which is why the paper asks the tubes, the order of draw and the rules for each specimen.

The universals: identify the patient by two identifiers (name and hospital number or date of birth) against the request, explain and take consent, wear gloves and follow standard precautions, use the container the test needs, fill it to the mark, label it at the bedside immediately after collection — never before — with name, number, date, time and site, seal it in a biohazard bag with the requisition outside, send it promptly, and record what was sent and when; for a medico-legal sample the chain of custody is documented at every hand-over.
Venepuncture: the median cubital vein at the antecubital fossa with a tourniquet for no more than a minute, a 21- or 22-gauge needle, the skin cleaned and dried, never the arm with an intravenous infusion, a fistula, lymphoedema after mastectomy, a haematoma or a burn; tubes are inverted gently five to eight times, never shaken, because haemolysis releases potassium and lactate dehydrogenase and gives a falsely high potassium; fasting samples need 8 to 12 hours without food.
The order of draw and the tubes: blood culture bottles first (sterile), then the light-blue sodium citrate tube (coagulation — prothrombin time and activated partial thromboplastin time, filled exactly to the line for the 9:1 ratio), then plain red or gold serum-separator tubes (biochemistry, serology), then green heparin (electrolytes, some chemistry), then lavender EDTA (full blood count, ESR, HbA1c, blood grouping — EDTA clots must be rejected), then grey fluoride-oxalate (glucose — fluoride stops the cells consuming it), and black citrate for the Westergren ESR; the colour of the cap is the anticoagulant, and the wrong one spoils the test.
Special draws: arterial blood gas from the radial artery after Allen's test, in a heparinised syringe with every air bubble expelled, capped, on ice, analysed within 15 to 30 minutes, the site pressed for five minutes; blood culture before the first antibiotic dose, two sets from two sites after chlorhexidine-alcohol skin preparation, 8 to 10 mL per bottle in an adult and 1 to 3 mL in a child, aerobic bottle first, never refrigerated; capillary samples from the side of a fingertip or the lateral heel of a neonate, never the centre of the heel.
Urine: a first-morning midstream clean-catch sample of 10 to 50 mL in a sterile container for routine examination and culture, tested within one to two hours or refrigerated at 4 °C for up to 24 hours (bacteria multiply, casts dissolve, glucose falls and pH rises in a warm sample); from a catheter the sample is taken with a syringe from the sampling port after clamping, never from the drainage bag; a 24-hour collection discards the first void, collects every void including the one at the finishing time, with the preservative the laboratory supplies; suprapubic aspiration is the reference in infants and the collection bag the practical alternative.
Stool, sputum and swabs: stool of about 5 to 10 g (a walnut) or 5 mL if liquid in a clean dry container uncontaminated by urine or water, within an hour for trophozoites (which die and lose motility), three samples on separate days for ova and cysts, Cary-Blair medium for culture, and no red meat, vitamin C or NSAIDs for three days before an occult blood test; sputum is an early-morning deep-cough specimen of 2 to 5 mL of mucopurulent material after rinsing the mouth with water, not saliva, in a wide-mouthed container, two samples (spot and early morning) for acid-fast bacilli; throat, wound and pus swabs are taken before antibiotics from the base of the lesion, placed in transport medium and sent within two hours, and an aspirate of pus in a syringe beats a swab every time.
Cerebrospinal fluid is taken by the doctor at lumbar puncture between L3-L4 or L4-L5 with the patient curled on the side, into three or four numbered sterile tubes — biochemistry (protein, glucose with a simultaneous blood glucose), microbiology (Gram stain, culture, acid-fast stain) and cell count, with a fourth for cytology or special tests — and carried by hand to the laboratory at room temperature, never refrigerated, because the meningococcus and the pneumococcus die in the cold; normal fluid is crystal clear with 0 to 5 lymphocytes, protein 15 to 45 mg/dL, glucose 40 to 70 mg/dL (about two-thirds of the blood value) and an opening pressure of 60 to 180 mm of water; tissue for histology goes into 10 per cent formalin of ten times its volume, tissue for frozen section fresh and dry, and tissue for culture in sterile saline; samples are rejected when unlabelled or mislabelled, haemolysed, clotted, insufficient, leaking, in the wrong container or too old.

Urine examination — physical, chemical and microscopic

Urine examination is the routine three-part test — physical, chemical and microscopic — that reads the kidney, the urinary tract and a good deal of the body's metabolism from a sample that costs nothing to obtain: its colour and specific gravity, the dipstick's protein, glucose, ketones, blood, bilirubin, urobilinogen, nitrite and leucocyte esterase, and the sediment's cells, casts, crystals and organisms. It is asked in every paper for the normal values and the meaning of each abnormal finding, and the ward nurse performs the dipstick herself, so the technique and its errors are hers too.

Physical examination: volume of 800 to 2000 mL a day (oliguria below 400 mL, anuria below 100 mL, polyuria above 2500 to 3000 mL); colour pale yellow from urochrome — dark when concentrated or with bilirubin, red or smoky with blood, haemoglobin or myoglobin (and with beetroot or rifampicin, which turns it orange), cola-coloured in post-streptococcal glomerulonephritis and rhabdomyolysis, milky with pus or chyle; turbidity from pus, bacteria, phosphates or crystals; an ammoniacal smell in an old or infected sample, a fruity smell from ketones, a mousy smell in phenylketonuria.
Specific gravity of 1.003 to 1.030 measures concentrating power — low in diabetes insipidus and after fluids, high in dehydration, glycosuria and contrast, fixed at 1.010 (isosthenuria) when the failing kidney can no longer concentrate or dilute; pH 4.5 to 8, usually about 6 — acidic with a meat diet, acidosis and most urinary infections, alkaline with a vegetarian diet, alkalosis, a Proteus infection that splits urea, and a sample that has stood too long.
Protein: the normal loss is under 150 mg a day and the dipstick, which detects mainly albumin, reads trace, then 1+ (about 30 mg/dL), 2+ (100), 3+ (300) and 4+ (1000 or more); nephrotic-range proteinuria is above 3.5 g a day, microalbuminuria of 30 to 300 mg a day is the first sign of diabetic nephropathy and needs a special strip or the albumin-creatinine ratio, orthostatic, febrile and exercise proteinuria are benign, and Bence Jones protein is missed by the dipstick and needs the heat test or electrophoresis.
Glucose appears when the blood level passes the renal threshold of about 180 mg/dL — diabetes, pregnancy with a lowered threshold, renal glycosuria; Benedict's test grades the reducing sugar by colour from blue (nil) through green (0.5 per cent), yellow, orange to brick red (2 per cent or more) and is fooled by other reducing substances and vitamin C, while the dipstick's glucose oxidase is specific for glucose; ketones (Rothera's test — a purple ring) mean fat is being burnt — diabetic ketoacidosis, starvation, vomiting, a low-carbohydrate diet — and the dipstick detects acetoacetate, not beta-hydroxybutyrate.
Blood on the strip is haematuria, haemoglobinuria or myoglobinuria and only the microscope tells them apart (red cells present only in haematuria); bilirubin in urine is always conjugated and means hepatic or obstructive jaundice (the foam test and Fouchet's test), urobilinogen rises in haemolysis and hepatitis and disappears in complete biliary obstruction; nitrite means a Gram-negative organism has had about four hours in the bladder to reduce nitrate, and leucocyte esterase means pus cells — together they screen for infection.
The sediment: red cells above 2 or 3 per high-power field are abnormal and dysmorphic red cells or red-cell casts mean the glomerulus is bleeding; white cells above 5 per field are pyuria, and pyuria with a sterile culture points to tuberculosis, chlamydia, stones or analgesic nephropathy; squamous epithelial cells mean a contaminated sample; casts are moulded in the tubules — hyaline in dehydration and after exercise (normal in small numbers), red-cell casts in glomerulonephritis, white-cell casts in pyelonephritis, granular casts in acute tubular necrosis and chronic disease, fatty casts in the nephrotic syndrome, and broad waxy casts in chronic renal failure.
Crystals are calcium oxalate (envelopes — usually normal, stones, ethylene glycol), uric acid (rhomboids — gout, tumour lysis), triple phosphate (coffin lids — Proteus infection and struvite stones) and cystine (hexagons — cystinuria); a culture is significant at 100,000 colony-forming units per mL of a midstream sample and at any growth from a suprapubic aspirate; and the nurse's technique decides all of it — a first-morning sample tested within an hour, the strip dipped and the excess tapped off, each pad read at its stated time against the chart in good light, the bottle capped tightly, and the result written with the time.

Biopsy, FNAC, Pap smear and normal laboratory values

A biopsy is the removal of living tissue for histopathological examination, fine-needle aspiration cytology is the study of cells sucked from a lump through a thin needle, and the Papanicolaou smear is the study of cells scraped from the cervix to find the precancerous change before it becomes a cancer — three ways of putting a diagnosis under the microscope, each with a technique the nurse assists or performs and a fixative that decides whether the sample can be read. The chapter closes with the normal laboratory values every nurse is examined on and works by, and the critical values that are phoned rather than filed.

Biopsy by type: incisional (a piece of a large lesion), excisional (the whole lesion with a margin — the treatment as well as the diagnosis), punch (a 3 to 4 mm cylinder of skin), core or needle (a Tru-cut core from breast, prostate, liver or kidney, often under ultrasound), endoscopic, bone marrow trephine, and the frozen section — fresh tissue frozen and cut in the operating theatre for an answer in minutes, sent dry in a labelled container with no fixative; every other specimen goes into 10 per cent neutral buffered formalin of at least ten times its volume, never saline, with the site, orientation sutures and inked margins recorded, and tissue for culture goes separately in sterile saline.
The laboratory then processes, embeds, sections and stains — haematoxylin and eosin for everything, Ziehl-Neelsen for acid-fast bacilli, periodic acid-Schiff for fungi and glycogen, Congo red for amyloid (apple-green under polarised light), Perls' Prussian blue for iron, Masson's trichrome for collagen, Giemsa and Gram for organisms — and immunohistochemistry for the receptors and markers (oestrogen, progesterone and HER2 in breast cancer) that choose the drug; a histology report takes days, and the nurse who tells the patient 'the result will be back tomorrow' has promised what the laboratory cannot deliver.
Fine-needle aspiration cytology uses a 22- to 23-gauge needle on a 10 mL syringe, without anaesthesia, on thyroid, breast, lymph node, salivary gland and any palpable lump, and under ultrasound or CT for deep lesions; the aspirate is smeared thinly on slides, half air-dried for Giemsa and half fixed at once in 95 per cent alcohol for the Papanicolaou stain; it is quick, cheap and safe (a small haematoma, rarely a pneumothorax from the lung) and reads cells but not architecture — it cannot tell a follicular adenoma from a follicular carcinoma of the thyroid because that needs capsular invasion, and a negative aspirate from a suspicious lump is followed by a biopsy.
The Pap smear screens for cervical intraepithelial neoplasia in women from 21 to 25 years to 65, every three years by cytology or every five with HPV testing, with visual inspection with acetic acid as India's programme method at 30 to 65; the woman is not menstruating and has had no intercourse, douching or vaginal medication for 48 hours; in the lithotomy position an unlubricated speculum (water only) exposes the cervix, the Ayre's spatula is rotated through 360 degrees at the squamocolumnar junction and the endocervical brush is added, the material is spread thinly on a labelled slide and fixed immediately — in 95 per cent ethyl alcohol or with spray — before it can air-dry, or rinsed into the vial for liquid-based cytology.
The report follows the Bethesda system: negative for intraepithelial lesion or malignancy; atypical squamous cells of undetermined significance or that cannot exclude a high-grade lesion; low-grade squamous intraepithelial lesion (CIN 1, HPV effect); high-grade squamous intraepithelial lesion (CIN 2 and 3); atypical glandular cells; and carcinoma — anything from ASC-H upward goes to colposcopy and biopsy, HPV vaccination of girls at 9 to 14 prevents the lesion in the first place, and a woman who has had a hysterectomy for a benign cause needs no further smears.
Haematology normal values (adult): haemoglobin 13 to 17 g/dL in men and 12 to 15 in women; red cells 4.5 to 5.5 and 4 to 5 million per microlitre; packed cell volume 40 to 50 and 36 to 46 per cent; MCV 80 to 100 fL, MCH 27 to 32 pg, MCHC 32 to 36 g/dL; white cells 4,000 to 11,000 per microlitre with neutrophils 40 to 75 per cent, lymphocytes 20 to 45, monocytes 2 to 10, eosinophils 1 to 6 and basophils up to 1; platelets 150,000 to 450,000; reticulocytes 0.5 to 2 per cent; ESR by Westergren 0 to 15 mm in men and 0 to 20 in women in the first hour; bleeding time 2 to 7 minutes, clotting time 8 to 15 minutes, prothrombin time 11 to 13.5 seconds (INR 0.8 to 1.2, warfarin target 2 to 3), activated partial thromboplastin time 25 to 35 seconds (heparin target 1.5 to 2.5 times control), fibrinogen 200 to 400 mg/dL.
Biochemistry normal values (adult): fasting glucose 70 to 99 mg/dL (100 to 125 impaired, 126 or more diabetes), HbA1c below 5.7 per cent (6.5 or more diabetes); urea 15 to 40 mg/dL, creatinine 0.6 to 1.2 mg/dL, uric acid 3.5 to 7.2 mg/dL; sodium 135 to 145, potassium 3.5 to 5.0, chloride 96 to 106 and bicarbonate 22 to 26 mEq/L; calcium 8.5 to 10.5 mg/dL, magnesium 1.7 to 2.2 mg/dL, phosphate 2.5 to 4.5 mg/dL; total protein 6 to 8 g/dL, albumin 3.5 to 5 g/dL, total bilirubin 0.2 to 1.2 mg/dL (direct below 0.3), AST 10 to 40 U/L, ALT 7 to 56 U/L, alkaline phosphatase 40 to 120 U/L; total cholesterol under 200 mg/dL desirable, LDL under 100, HDL above 40 in men and 50 in women, triglycerides under 150; TSH 0.4 to 4.0 mIU/L; arterial blood gas pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg, SaO₂ 95–100 %, anion gap 8–12 mEq/L (12–16 mEq/L when K⁺ is included); and the critical values that are telephoned at once — potassium below 2.5 or above 6.5, sodium below 120 or above 160, glucose below 40 or above 500, haemoglobin below 6, platelets below 20,000, an INR above 5, a positive blood culture or cerebrospinal fluid Gram stain and a positive troponin.

Free today: the 10-question daily quiz · the NORCET 11 recall paper · what ₹999 unlocks — once, no auto-renewal.