📚 Urine Analysis: From Renal Physiology to Diagnostic Testing | 尿液分析：从肾脏生理到诊断检测
Urine analysis is a simple, non-invasive investigation that links the physiology of the kidney to clinical diagnosis. In Cambridge A Level Biology, understanding how urine is produced – by ultrafiltration, selective reabsorption and osmoregulation – helps explain why changes in urine composition can indicate diabetes, kidney disease, infections, pregnancy or drug use. This article examines the principles, methods and interpretation of urine analysis.
尿液分析是一种简单、无创的检查方法，它将肾脏生理与临床诊断联系起来。在剑桥 A Level 生物课程中，理解尿液如何通过超滤、选择性重吸收和渗透调节生成，有助于解释为什么尿液成分的变化可以提示糖尿病、肾脏疾病、感染、妊娠或药物使用。本文探讨尿液分析的原理、方法和结果解读。
1. Overview of Urine Formation | 尿液形成概述
Urine is produced by the kidneys through three main processes: ultrafiltration in the glomerulus, selective reabsorption in the proximal convoluted tubule and loop of Henle, and secretion in the distal convoluted tubule and collecting duct. Blood enters the glomerulus through the afferent arteriole. The high hydrostatic pressure forces water and small solutes through the capillary endothelium, basement membrane and podocytes into Bowman’s capsule. This filtrate contains glucose, urea, ions and water, but normally not blood cells or large plasma proteins.
尿液由肾脏通过三个主要过程产生：肾小球中的超滤、近曲小管和髓袢中的选择性重吸收，以及远曲小管和集合管中的分泌。血液经入球小动脉进入肾小球。较高的静水压迫使水和小分子溶质依次通过毛细血管内皮、基底膜和足细胞进入鲍曼氏囊。滤液中含有葡萄糖、尿素、离子和水，但正常情况下不含血细胞或大分子血浆蛋白。
As the filtrate moves along the nephron, useful substances such as glucose, amino acids, most ions and most water are reabsorbed into the blood. Waste products such as urea remain in the tubule. The final urine passes through the ureters to the bladder, where it is stored before urination. Urine analysis detects the presence or concentration of substances that should normally be absent, reduced or increased.
当滤液沿肾单位流动时，葡萄糖、氨基酸、大多数离子和大部分水等有用物质被重新吸收回血液。尿素等废物则留在肾小管中。最终尿液经输尿管进入膀胱，在排尿前储存。尿液分析可检测本应不存在、减少或增加的物质的浓度。
2. The Nephron and Urine Composition | 肾单位与尿液成分
The functional unit of the kidney is the nephron. Normal urine is approximately 95% water. Its main solutes are urea, creatinine, uric acid, ammonia and inorganic ions such as Na⁺, K⁺, Cl⁻, phosphate and sulfate. Small amounts of urobilinogen give urine its yellow colour. In a healthy person, urine should not contain detectable glucose, protein, blood, ketones or nitrites.
肾脏的功能单位是肾单位。正常尿液约 95% 是水。其主要溶质包括尿素、肌酐、尿酸、氨以及 Na⁺、K⁺、Cl⁻、磷酸盐和硫酸盐等无机离子。少量尿胆素原使尿液呈黄色。在健康人体内，尿液中不应检出葡萄糖、蛋白质、血液、酮体或亚硝酸盐。
| Substance / 物质 | Normal finding / 正常结果 | Clinical significance if abnormal / 异常时的临床意义 |
|---|---|---|
| Glucose / 葡萄糖 | Absent / 无 | Diabetes mellitus, pregnancy / 糖尿病、妊娠 |
| Protein / 蛋白质 | Absent or trace / 无或微量 | Glomerular damage, hypertension / 肾小球损伤、高血压 |
| Red blood cells / 红细胞 | Absent / 无 | Infection, stones, glomerulonephritis / 感染、结石、肾小球肾炎 |
| Ketones / 酮体 | Absent / 无 | Diabetic ketoacidosis, starvation / 糖尿病酮症酸中毒、饥饿 |
| Nitrites / 亚硝酸盐 | Absent / 无 | Bacterial infection / 细菌感染 |
| hCG / 人绒毛膜促性腺激素 | Absent in non-pregnant / 非妊娠时无 | Pregnancy, certain tumours / 妊娠、某些肿瘤 |
3. Collection and Handling of Urine Specimens | 尿液样本的采集与处理
To avoid contamination, a mid-stream urine (MSU) sample is usually collected. The patient begins to urinate, stops briefly, then collects the middle portion of urine in a sterile container. This method reduces contamination by bacteria and epithelial cells from the urethra and external genitalia. The first portion of urine flushes away these contaminants.
为避免污染，通常采集中段尿（MSU）样本。患者先开始排尿，稍作停顿，然后用无菌容器收集中间部分的尿液。此方法可减少来自尿道和外生殖器的细菌及上皮细胞污染。最初排出的尿液可冲洗掉这些污染物。
Urine should be analysed promptly or refrigerated because bacteria multiply rapidly at room temperature and may change pH, glucose and nitrite results. For pregnancy testing, first morning urine is often recommended because it is most concentrated and therefore contains the highest concentration of hCG. For quantitative tests, a 24-hour urine collection may be used to measure total protein or creatinine clearance.
尿液应尽快检测或冷藏，因为细菌在室温下迅速繁殖，可能改变 pH、葡萄糖和亚硝酸盐的结果。妊娠试验通常建议使用晨尿，因为晨尿最浓缩，hCG 浓度最高。对于定量检测，可使用 24 小时尿液收集来测定总蛋白或肌酐清除率。
4. Physical Examination: Colour, Clarity and Odour | 物理检查：颜色、透明度与气味
Normal urine colour ranges from pale straw to amber, mainly due to the pigment urochrome. Dark urine often indicates concentrated urine from dehydration, while very pale urine may be due to high fluid intake or diabetes insipidus. Red or brown urine may indicate haematuria, haemoglobinuria or myoglobinuria. Orange-brown urine may contain bile pigments such as bilirubin.
正常尿液颜色从淡稻草色到琥珀色不等，主要由于尿色素所致。深色尿液常提示脱水导致的浓缩尿，而非常浅的尿液可能因大量饮水或尿崩症引起。红色或棕色尿液可能提示血尿、血红蛋白尿或肌红蛋白尿。橙棕色尿液可能含有胆红素等胆色素。
Cloudy urine can be caused by infection, white blood cells, mucus, phosphates or crystals. A sweet or fruity odour may indicate ketones, especially in diabetic ketoacidosis. An ammonia-like smell can develop if urine is left standing because bacteria convert urea to ammonia. Foods and medicines can also change colour, for example beetroot may produce red urine.
尿液混浊可能由感染、白细胞、黏液、磷酸盐或晶体引起。甜味或水果味可能提示酮体存在，尤其在糖尿病酮症酸中毒时。尿液放置过久会产生氨味，因为细菌将尿素转化为氨。食物和药物也可改变颜色，例如甜菜根可使尿液呈红色。
5. Chemical Dipstick Tests: Glucose and Ketones | 化学试纸检测：葡萄糖与酮体
Dipstick tests use immobilised enzymes and chemical indicators that change colour in the presence of specific substances. The glucose test uses glucose oxidase and peroxidase. Glucose oxidase oxidises glucose to gluconic acid and hydrogen peroxide, and the peroxide then reacts with a chromogen to produce a colour change proportional to glucose concentration.
尿液试纸检测使用固定化酶和化学指示剂，在特定物质存在时产生颜色变化。葡萄糖检测使用葡萄糖氧化酶和过氧化物酶。葡萄糖氧化酶将葡萄糖氧化为葡萄糖酸和过氧化氢，过氧化物再与色原反应，产生与葡萄糖浓度成比例的颜色变化。
Normally, all filtered glucose is reabsorbed in the proximal convoluted tubule by sodium-glucose co-transporters. However, if blood glucose exceeds the renal threshold of about 10 mmol dm⁻³, these carriers become saturated and glucose appears in the urine, a condition called glycosuria. Glycosuria is commonly caused by diabetes mellitus but may also occur in pregnancy or when renal tubular reabsorption is impaired.
正常情况下，所有滤过的葡萄糖都在近曲小管通过钠-葡萄糖协同转运蛋白被重吸收。但如果血糖超过约 10 mmol dm⁻³ 的肾阈，这些载体达到饱和，葡萄糖便出现在尿液中，称为糖尿。糖尿通常由糖尿病引起，但也可见于妊娠或肾小管重吸收受损时。
Ketones – acetoacetate, acetone and beta-hydroxybutyrate – appear in urine when fatty acid metabolism increases. This occurs in diabetic ketoacidosis, starvation, prolonged vomiting or very low carbohydrate diets. The dipstick reaction is based on sodium nitroprusside, which reacts mainly with acetoacetate and acetone.
酮体——乙酰乙酸、丙酮和 β-羟丁酸——在脂肪酸代谢增强时出现在尿液中。这种情况可见于糖尿病酮症酸中毒、饥饿、长期呕吐或极低碳水化合物饮食。试纸反应基于硝普钠，主要与乙酰乙酸和丙酮反应。
6. Proteinuria and Albuminuria | 蛋白尿与白蛋白尿
The glomerular filtration barrier normally prevents most plasma proteins from entering the filtrate. This barrier is selective because of the size of the pores and the negative charge of the basement membrane and podocytes, which repels negatively charged albumin. Any small amount of albumin that does pass is usually reabsorbed and broken down by proximal tubule cells.
肾小球滤过屏障通常阻止大多数血浆蛋白进入滤液。该屏障具有选择性，因为孔隙的大小以及基底膜和足细胞带负电荷，可排斥带负电荷的白蛋白。少量通过的白蛋白通常也会被近曲小管细胞重吸收并分解。
Significant protein in the urine, called proteinuria, indicates increased glomerular permeability or impaired tubular reabsorption. Causes include glomerulonephritis, diabetic nephropathy, hypertension and pre-eclampsia. Dipsticks mainly detect albumin; they may miss other proteins such as Bence Jones proteins. Microalbuminuria – a small increase in albumin – is an early marker of diabetic kidney disease.
尿中出现显著蛋白质称为蛋白尿，提示肾小球通透性增加或肾小管重吸收受损。病因包括肾小球肾炎、糖尿病肾病、高血压和子痫前期。试纸主要检测白蛋白，可能漏检本周蛋白等其他蛋白质。微量白蛋白尿——白蛋白轻度升高——是糖尿病肾病的早期标志。
7. Urinary pH, Specific Gravity and Osmolality | 尿液 pH、比重与渗透压
Normal urine pH is usually around 6.0 but can range from about 4.5 to 8.0. Diet influences pH: a high-protein diet lowers pH, while a diet rich in vegetables raises it. Persistently alkaline urine may occur in urinary tract infections caused by urease-producing bacteria such as Proteus, because urease converts urea into ammonia, which raises pH.
正常尿液 pH 通常约为 6.0，但可在约 4.5 至 8.0 之间变化。饮食会影响 pH：高蛋白饮食降低 pH，富含蔬菜的饮食升高 pH。持续性碱性尿可见于产脲酶细菌（如变形杆菌）引起的尿路感染，因为脲酶将尿素转化为氨，使 pH 升高。
Specific gravity (SG) measures the concentration of solutes in urine. Normal SG is about 1.003 to 1.030. Low specific gravity may indicate diabetes insipidus, where ADH is absent or ineffective, or excessive fluid intake. High specific gravity may result from dehydration, glycosuria or the syndrome of inappropriate ADH secretion. Osmolality is a more accurate measure of solute concentration.
比重（SG）测量尿液中溶质的浓度。正常比重约为 1.003 至 1.030。低比重可能提示尿崩症，即抗利尿激素（ADH）缺乏或无效，或饮水过多。高比重可能由脱水、糖尿或抗利尿激素分泌不当综合征引起。渗透压是衡量溶质浓度更准确的指标。
Urine concentration is regulated by ADH. When blood water potential falls, osmoreceptors in the hypothalamus trigger ADH release from the posterior pituitary. ADH binds to receptors on collecting duct cells and causes insertion of aquaporin-2 water channels, increasing water reabsorption and producing a smaller volume of more concentrated urine.
尿液浓度受 ADH 调节。当血液水势下降时，下丘脑中的渗透压感受器触发垂体后叶释放 ADH。ADH 与集合管细胞上的受体结合，促使水通道蛋白-2 插入细胞膜，增加水的重吸收，从而产生量更少、浓度更高的尿液。
8. Microscopic Examination: Cells, Casts and Crystals | 显微镜检查：细胞、管型与晶体
Microscopic examination is performed on the sediment obtained by centrifuging urine. Red blood cells may indicate glomerulonephritis, kidney stones, trauma or infection. White blood cells suggest inflammation or infection of the urinary tract, such as cystitis or pyelonephritis. Epithelial cells may be due to contamination or tubular damage.
显微镜检查是对尿液离心后获得的沉渣进行的。红细胞可能提示肾小球肾炎、肾结石、创伤或感染。白细胞提示尿路炎症或感染，如膀胱炎或肾盂肾炎。上皮细胞可能由污染或肾小管损伤引起。
Casts are cylindrical structures formed in the distal tubules and collecting ducts from Tamm-Horsfall protein. Hyaline casts may be normal after exercise or dehydration. Red cell casts are strongly associated with glomerulonephritis; white cell casts suggest pyelonephritis; granular casts indicate acute tubular injury. Crystals such as calcium oxalate, uric acid, cystine and struvite may be normal or may indicate a risk of kidney stones depending on urine pH and concentration.
管型是在远曲小管和集合管中由 Tamm-Horsfall 蛋白形成的圆柱状结构。透明管型在运动或脱水后可能正常。红细胞管型与肾小球肾炎密切相关；白细胞管型提示肾盂肾炎；颗粒管型提示急性肾小管损伤。草酸钙、尿酸、胱氨酸和鸟粪石等晶体可能是正常的，也可能提示肾结石风险，取决于尿液 pH 和浓度。
9. Hormone Detection: hCG and Pregnancy Testing | 激素检测：hCG 与妊娠测试
Pregnancy tests detect human chorionic gonadotropin (hCG) in urine. After implantation of the embryo, trophoblast cells secrete hCG, which enters the maternal blood and is filtered into the urine. The concentration of hCG rises rapidly in early pregnancy, doubling approximately every two days.
妊娠试验检测尿液中的人绒毛膜促性腺激素（hCG）。胚胎着床后，滋养层细胞分泌 hCG，hCG 进入母体血液并被滤入尿液。妊娠早期 hCG 浓度迅速升高，约每两天
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